The present invention relates to a variable geometry turbine. The variable geometry turbine may, for example, form a part of a turbocharger. This application claims priority from the following British patent applications GB0917513.4, GB1005680.2, GB1012382.6, GB1012389.1, GB1012488.1, GB1012474.1, GB1012536.7, GB1012734.8, GB1012557.3, GB1012767.8, GB1012769.4, GB1012463.4, GB1012471.7, GB1012475.8, GB1012479.0, GB1012492.3, GB1012774.4, GB1012715.7, GB1012538.3, GB1012658.9, GB1012486.5, GB1012768.6, GB1012779.3, GB1012380.0, and GB1012744.7, which all relate to a variable geometry turbine, the entire contents of each of which is hereby incorporated by reference. Turbochargers are well known devices for supplying air to an intal^e of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings. The turbine stage of a typical turbocharger comprises: a turbine chamber within which the turbine wheel is mounted; an annular inlet defined between facing radial walls arranged around the turbine chamber; an inlet volute arranged around the annular inlet; and an outlet passageway extending from the turbine chamber. The passageways and chamber communicate such that pressurised exhaust gas admitted to the inlet volute flows through the inlet to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet so as to deflect gas flowing through the inlet. That is, gas flowing through the annular inlet flows through inlet passages (defined between adjacent vanes) which induce swirl in the gas flow, turning the flow direction towards the direction of rotation of the turbine wheel. Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that the size of the inlet can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the inlet using a variable geometry mechanism. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers. Nozzle vane arrangements in variable geometry turbochargers can take different forms. In one type, known as a "sliding nozzle ring", the vanes are fixed to an axially movable wall that slides across the inlet passageway. The axially movable wall moves towards a facing shroud plate in order to close down the inlet passageway and in so doing the vanes pass through apertures in the shroud plate. Alternatively, the nozzle ring is fixed to a wall of the turbine and a shroud plate is moved over the vanes to vary the size of the inlet passageway. The moving component of the variable geometry mechanism, whether it is the nozzle ring or the shroud plate, is supported for axial movement in a cavity in a part of the turbocharger housing (usually either the turbine housing or the turbocharger bearing housing). It may be sealed with respect to the cavity walls to reduce or prevent leakage flow around the back of the nozzle ring. The moveable wall of the variable geometry mechanism is axially displaced by a suitable actuator assembly comprising an actuator and a linkage. An example of such a known actuator assembly is for instance disclosed in US 5,868,552. The linkage comprises a yoke pivotally supported within the bearing housing and having two arms, each of which extends into engagement with an end of a respective push rod on which the moving component (in this instance the nozzle ring) is mounted. The yoke is mounted on a shaft journaled in the bearing housing and supporting a crank external to the bearing housing which may be connected to the actuator in any appropriate manner. The actuator which moves the yoke can take a variety of forms, including pneumatic, hydraulic and electric forms, and can be linked to the yoke in a variety of ways. The actuator will generally adjust the position of the moving wall under the control of an engine control unit (ECU) in order to modify the airflow through the turbine to meet performance requirements. 3 In use, axial forces are imported on the moveable wall by the air flow through the Inlet, which must be accommodated by the actuator assembly. In addition, a torque is imparted to the nozzle ring as a result of gas flow vane passages being deflected towards the direction of rotation of the turbine wheel. If the nozzle ring is the moving wall of the variable geometry mechanism this torque also has to be reacted or othenwise accommodated by the actuator assembly such as by parts of the linkage. It is one object of the present invention to obviate or mitigate the aforesaid disadvantages. It is also an object of the present invention to provide an improved or alternative variable geometry mechanism and turbine. Statements of Invention According to an aspect of the present invention there is provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into at least two axially offset inlet passages, and wherein the inner diameter of the sleeve is greater than the inner diameter of the inlet passages. It will be appreciated that axially offset inlet passages include inlet passages with different axial positions and/or inlet passages with different axial extents. Axially offset inlet passages may be spaced apart, adjacent or axially overlapping. Moreover, it will be appreciated that references to the sleeve as being 'cylindrical' are to be interpreted as encompassing any generally cylindrical or annular shape, and does not exclude sleeves having a structure which lacks a perfectly circular axial cross-section. By way of example, cylindrical sleeves in accordance with the present invention may include sections or segments which are not perfectly arcuate so as to define a continuously circular axial cross-section. Such sections or segments could, for example, be substantially straight in axial cross section provided a sufficient number are provided to define a generally cylindrical sleeve. The inner diameter of the sleeve may be less than or substantially equal to the outer diameter of the inlet passages. Alternatively, the inner diameter of the sleeve is greater than the outer diameter of the inlet passages. As a further alternative, the turbine may incorporate a plurality of axially movable sleeves, a first sleeve having an inner diameter that is greater than the inner diameter of the inlet passages, and a second sleeve having an inner diameter that is less than or substantially equal to the outer diameter of the inlet passages or an inner diameter that is greater than the outer diameter of the inlet passages. In a still further embodiment, the turbine may incorporate a plurality of axially movable sleeves, a first sleeve having an inner diameter that is less than or substantially equal to the outer diameter of the inlet passages, and a second sleeve having an inner diameter that is greater than the outer diameter of the inlet passages. The sleeve may be axially movable across the annular inlet in a direction towards the second inlet sidewall so as reduce the size of the gas flow path through the inlet. At least a portion of an end of the sleeve nearer to the first inlet sidewall than the second inlet sidewall may be configured so as to be exposable to gases flowing through said annular inlet during use. Additionally or alternatively, at least a portion of an end of the sleeve nearer to the first inlet sidewall than the second inlet sidewall may be configured so as to be located in between said first and second inlet sidewall during axial movement of the sleeve across the annular inlet. The sleeve preferably possesses a small radial thickness or extent, which may, for example, be less than the axial width of the annular inlet. This is intended to reduce aerodynamic load on the sleeve, or actuators thereof. 'Small', may be defined as being less than an axial width of the annular inlet, or less than an axial width of an inlet portion or passage way. The sleeve may be less than 5mm thick, less than 4mm thick, less than 3mm thick, less than 2mm thick, or less than 1mm thick, for example approximately 0.5mm thick. The annular inlet may be divided into at least two axially offset inlet passages by at least one annular baffle axially spaced from the first and second inlet sidewalls. Inlet vanes may extend axially across at least one of the axially offset inlet passages. The minimum distance between a baffle and tlie turbine wheel may be less than the minimum distance between an adjacent vane and the turbine wheel. The trailing edges of at least some of the vanes extending across one of the axially offset inlet passages may lie on a different radius to the trailing edges of at least some of the vanes extending across another of the axially offset inlet passages. The annular inlet may be divided into an annular array of substantially tubular inlet passages extending generally towards the turbine wheel, wherein the annular array of inlet passages comprises at least three axially offset inlet passages. A variable geometry turbine may comprise an annular inlet surrounding a turbine wheel mounted for rotation about a turbine axis within a turbine chamber defined by a housing, the chamber having an annular inlet defined between inboard and outboard inlet side walls and surrounding the turbine wheel, the annular inlet including: a first pair of first and second circumferentially spaced inlet passages; and a second pair of third and fourth circumferentially spaced inlet passages; wherein the second pair of inlet passages is axially displaced from the first pair of inlet passages; and wherein a cylindrical sleeve is supported within the housing for reciprocal motion in an axial direction to vary the size of the annular inlet; and wherein the sleeve is movable between at least a first position in which each of the first pair of inlet passages Is at least partially open to gas flow, and the second pair of inlet passages are fully open to gas flow, and a second position In which the first pair of inlet passages are fully blocked to the gas flow and each of the second pair or inlet passages is at least partially blocked to gas flow. Typically, exhaust gas may flow to the annular inlet via a surrounding volute. In some embodiments of the invention the volute may be axially or circumferentially divided, the annular inlet being defined downstream of the volute or any divided portion of the volute. In such divided volute turbines the adjacent volute portions generally do not communicate with each other, other than at their downstream ends where they terminate at the inlet. The inboard and outboard inlet sidewalls may for instance be continuations of walls which define the volute. Typically, the maximum width of the inlet will correspond to the area swept out by rotation of the tips of the turbine wheel blades. When the sleeve is in the second position each of the second pair of inlet passages may be fully blocked to gas flow. Some variable geometry turbochargers may include a third pair of fourth and fifth circumferentially spaced inlet passages which are axially displaced from both the first and second pairs of inlet passages. Such embodiments may comprise four or more axially displaced pairs of circumferentially spaced inlet passages. When the sleeve is in the second position, all but one of said axially spaced pairs of circumferentially spaced inlet passages may be fully blocked to gas flow, the remaining pair of circumferentially spaced inlet passages being at least partially blocked to gas flow. Each of the pairs of inlet passages may be a part of a respective annular array of circumferentially spaced inlet passages suaounding the turbine wheel. Each pair or annular array of inlet passages may comprise passages which are substantially axially coincident. At least one inlet passage of at least one pair or annular array of inlet passages may axially overlap at least one of the inlet passages of an adjacent pair or annular array of inlet passages. The first position of the sleeve may be an open position in which each of said pairs or annular arrays of circumferentially spaced inlet passages are open to gas flow. The second position of the sleeve may be a closed position in which a free end of the sleeve projects across the annular inlet and abuts either the inboard or outboard side wall. The sleeve may be controllably positioned between said first and second positions. In some embodiments the number of inlet passages comprising each annular array of circumferentially spaced inlet passages may be the same. 1 In other embodiments the number of inlet passages comprising one annular array of circumferentially spaced inlet passages may differ from the number of inlet passages comprising at least one other annular array of circumferentially spaced inlet passages. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining a annular inlet surrounding the turbine wheel and defined between inboard and outboard inlet side walls, wherein a cylindrical sleeve is mounted within the housing for axial slideable movement across at least a portion of the annular inlet to vary the size of the annular inlet, further comprising: at least one annular baffle axlally spaced from the inboard and outboard side walls of the annular inlet to divide the annular inlet into axially adjacent annular portions, and wherein inlet vanes extend axially across at least two of said annular portions defined by the or each baffle. Again, gas may flow to the annular inlet via an annular volute or similar chamber surrounding the annular inlet. In some embodiments the volute may be a divided volute, for instance split into separate axial or circumferential portions which may for instance receive gas from different sources (e.g. different banks of cylinders in a multi-cylinder combustion engine). In embodiments of the present invention the inlet and baffle will be downstream of the volute, or any volute portions in a divided volute. A variable geometry turbine may comprise two or more axially spaced inlet baffles which axially divide the annular inlet into three or more annular regions, wherein inlet vanes extend across at least three of said annular regions. At least some inlet vanes may extend across the full width of the annular inlet between the inboard and outboard side walls. For instance, an annular an'ay of inlet vanes may extend across the annular inlet between the inboard and outboard side walls and two or more annular inlet baffles may be axially spaced within the annular inlet which together with the vanes define three or more axially spaced annular arrays of inlet passages. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing having an annular inlet surrounding the turbine wheel and defined between inboard and outboard inlet side walls, wherein the annular inlet is axially divided into adjacent annular regions by two or more annular inlet baffles, and wherein a cylindrical sleeve is mounted within the housing for axial slideable movement across at least a portion of the annular inlet to vary the size of the annular inlet. As with other variable geometry turbines, the annular inlet may be defined downstream of a surrounding volute (which may be a divided volute) or similar gas chamber. Inlet vanes may extend across at least one of the annular regions to divide the annular region into a circumferential spaced array of Inlet passages. Some variable geometry turbines which include inlet vanes as mentioned above, may be such that the trailing edges of at least a majority of vanes extending across an annular portion of the inlet may lie on a radius greater than the internal radius of a baffle defining the annular portion. In some variable geometry turbines all of the vanes extending across an annular portion of the inlet may have a trailing edge lying at a radius greater than the internal radius of a baffle defining the annular portion. In some embodiments each annular baffle may have an internal radius smaller than the radius of the leading edge of any vane of the annular inlet. The number of vanes extending across a first annular portion of the inlet may differ from the number of vanes extending across a second annular portion of the inlet. At least some of the vanes extending across a first annular portion of the inlet may have a configuration different to at least some of the vanes extending across a second annular portion of the inlet. For instance, the vanes extending across a first annular portion of the inlet may have a different swirl angle to the vanes extending across a second annular portion of the inlet. The trailing edges of at least some of the vanes extending across a first annular portion of the inlet may lie on a different radius to the trailing edges of at least some of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of all of the vanes extending across a first annular portion of the inlet lie on a radius different to that of the trailing edges of all of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of vanes of one annular portion of the inlet lie on a minimum radius which is different to that of vanes extending across any other annular portion of the inlet. Some variable geometry turbines may comprise at least two of said annular baffles which divide the annular inlet into at least three axially adjacent annular portions. Movement of the sleeve between positions defining the maximum and minimum width of the inlet is confined to discreet positions corresponding to the axial location of the or each annular baffle. Accordingly, in some variable geometry turbines the sleeve may be controlled to move in a step-wise fashion between discreet positions which may correspond to open and closed positions as well as intermediate positions, wherein each of the intermediate positions corresponds to the position of an annular baffle. In such intermediate positions the free end of the sleeve may axially align with the leading edge of a baffle. Some variable geometry turbines may comprise at least two of said annular baffles dividing the annular inlet into at least three axially adjacent annular portions, wherein at least one of said annular portions does not include any inlet vanes. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing including an annular inlet surrounding the turbine wheel and defined between inboard and outboard inlet side walls, wherein an annular array of inlet vanes extends between the inboard and outboard inlet side walls defining circumferentially spaced vane passages between adjacent inlet vanes, and wherein substantially circumferentially extending baffle walls extend between at least some adjacent pairs of inlet vanes to divide the respective vanes passages into axially spaced inlet passages. At least one baffle wall may be annular. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing including an annular inlet surrounding the turbine wtieel and defined between inboard and outboard inlet side walls, wherein the annular inlet includes a nozzle structure comprising an annular array of substantially tubular inlet passages extending generally towards the turbine wheel, wherein the annular array of inlet passages comprises at least three axially displaced inlet passages. The nozzle structure may be disposed downstream of an annular volute (which may be axially or circumferentially divided) which surrounds the annular inlet passage to deliver gas flow to the annular inlet passage. The inlet passages may have a generally diamond, pentagonal, hexagonal or other polygonal cross section along at least a portion of their length. In some variable geometry turbines the geometry of any given inlet passage may vary along its length. For instance, the cross-sectional area of the inlet passage may decrease to a minimum and then increase again. Similarly, the cross-sectional area may change shape at different positions along its length. For example, the inlet passage may have one cross section at its inlet (upstream) end and another cross section at its outlet (downstream) end. The cross section may change gradually along its length from inlet to outlet. Inlet passages may be substantially straight, or may be curved. In either case they may be swept fonwards or backwards relative to the direction of rotation of the turbine wheel. There may be two or more adjacent annular arrays of inlet passages. Adjacent annular an-ays may comprise inlet passages of a different number and/or size and/or geometry or configuration. For instance the passages of one annular array may define a different swirl angle to the passages of another annular an-ay. The inlet passages may be defined by two or more annular inlet baffles positioned within the annular inlet, wherein adjacent inlet baffles contact one another or are otherwise joined to one another at circumferentially spaced locations to define inlet passages between the areas of contact. The annular inlet baffles may be circumferentially corrugated, so that the areas of contact between adjacent baffles extend across substantially the full radial width of each annular baffle. The cylindrical sleeve of any aspect of the invention may be mounted within a housing cavity separated from the inlet passage by said inboard side wall, wherein a it free end of the cylindrical sleeve extends from said cavity into the annular inlet to define the width of the annular inlet. Gas flow through the annular inlet may therefore be confined between the free end of the sleeve and the outboard side wall. In some variable geometry turbines the housing comprises a bearing or centre housing portion, and a turbine housing portion, wherein the turbine wheel rotates in a chamber defined between the bearing/central housing and the turbine housing portions, and wherein the cylindrical sleeve is mounted with a housing cavity defined within the bearing/central housing. The cylindrical sleeve of any of the aspects of the invention may alternatively be mounted within a housing cavity separated from the inlet passage by said outboard side wall, wherein a free end of the cylindrical sleeve extends from said cavity into the annular inlet to define the width of the annular inlet. Gas flow through the annular inlet may therefore be confined between the free end of the sleeve and the inboard side wall. In some variable geometry turbines the housing comprises a bearing or centre housing portion, and a turbine housing portion, wherein the turbine wheel rotates in a chamber defined between the bearing/central housing and the turbine housing portions, and wherein the cylindrical sleeve is mounted with a housing cavity defined within the turbine housing. The cylindrical sleeve is preferably movable across an outside diameter of the annular inlet to selectively block upstream ends of respective inlet passages or portions relative to gas flow through the turbine. However in other variable geometry turbines the cylindrical sleeve is movable across an inside diameter of the annular inlet to selectively block downstream ends of respective inlet passages or portions relative to gas flow through the turbine. Preferentially, the sleeve surrounds the inlet portions, which has been found to give an improved aerodynamic performance. In other words, the inner diameter of the sleeve is greater than an outer diameter (or outer radial extent) of the inlet portion or fl- portions. In another embodiment, tiie sleeve may be surrounded by the inlet portions. In other words, the outer diameter of the sleeve may be less than inner diameter of the inlet portion or portions. In another embodiment, the sleeve may be moveable through the inlet portion or portions. In other words, the diameter (e.g. inner or outer, or average diameter) of the sleeve may be less than an outer diameter of the inlet portion or portions, and greater than an inner diameter of the inlet portion or portions. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between inboard and outboard inlet sidewalls, and further comprising at least one annular baffle axially spaced from the inboard and outboard sidewalls of the annular inlet to divide the annular inlet into axially adjacent annular portions, and a cylindrical sleeve axially movable within the annular inlet around the outside diameter of the annular inlet portions and said at least one annular baffle to vary the size of the annular inlet defined between a free end of the sleeve and either the inboard or outboard sidewall. Once again, the annular inlet may be defined downstream of a surrounding volute (including a divided volute or similar chamber for delivering gas flow to the annular inlet). The effective axial width of the inlet is defined between the free end of the sleeve and either the inboard or outboard sidewalls (depending on which side of the housing the sleeve is mounted). In some variable geometry turbines the cylindrical sleeve is mounted for movement in a step-wise manner between an open position, a closed position, and one or more positions corresponding to the position of the or each annular baffle. The sleeve is therefore constrained to move between discreet predetermined positions, some of which correspond to the location of inlet baffles. In some embodiments the sleeve may be prevented from being positioned such that its free end lies between adjacent baffles. One or more vanes may extend across at least one of the annular inlet portions. Accordingly, there may be provided a method of controlling or operating a turbine according to the present invention, in which the sleeve is moved in discreet axial steps between positions corresponding to a closed position, an open position and /5 intermediate positions in whicii tiie free end of tiie sleeve Is aligned with an annular inlet baffle. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet, at least a portion of the cylindrical sleeve being coated with a catalyst which promotes the decomposition of contaminants which pass through the inlet. The catalyst may promote the oxidation of soot. The annular inlet may be divided into at least two axially offset inlet passages. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, main inlet vanes and splitter inlet vanes extending axially into at least one of the first and second inlet portions, the main inlet vanes and splitter inlet vanes defining circumferentially adjacent inlet passages. It will be appreciated that axially offset inlet passages include inlet passages with different axial positions and/or inlet passages with different axial extents. Axially offset inlet passages may be spaced apart, adjacent or axially overlapping. The main inlet vanes and splitter vanes may extend in into at least one of the first and second inlet portions such that the main inlet vanes and splitter inlet vanes are circumferentially alternating. The radial distance between the turbine wheel and the trailing edge of a splitter inlet vane may be greater than the radial distance between the turbine wheel and the trailing edge of a main inlet vane. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turiDine wheel and defined between first and second inlet sidewalls, the annular inlet being divided into at least two axially offset inlet portions; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the sleeve is fomied from a rolled sheet of material. Opposing faces of the rolled sheet of material may be attached to one another, such that there is no overiap between ends of the sheet that carry the opposing faces. Opposing faces or ends of the roiled sheet of material may, instead, overiap one another. The sleeve, and/or a structure defining the inlet portions, is provided with an axially extending step. The step is a step up or down in the circumferential direction. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls, the annular inlet being divided into at least two axially offset inlet portions; a first cylindrical sleeve section axially movable across the annular inlet to vary the size of a gas flow path through the inlet; and a second cylindrical sleeve section axially movable across the annular inlet to vary the size of a gas flow path through the inlet. The first cylindrical sleeve section and the second cylindrical sleeve section may be independently moveable with respect to one another. The first cylindrical sleeve section and the second cylindrical sleeve section may be attached to one another. The first cylindrical sleeve section and the second cylindrical sleeve section may both have an inner diameter that is greater than an outer diameter of the inlet portions. The first cylindrical sleeve section and the second cylindrical sleeve section may both have an outer diameter that is less than an inner diameter of the inlet portions. The first cylindrical sleeve section may have an inner diameter that is greater than an outer diameter of the inlet portions; and/or the second cylindrical sleeve section may have an outer diameter that is less than an inner diameter of the inlet portions. The variable geometry turbine may further comprise a third cylindrical sleeve section, moveable to open or close a passage between the inlet, or a volume upstream of the inlet, and a turbine outlet. A variable geometry turbine may comprise: a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls, and a (e.g. substantially annular) baffle structure being axially moveable across the inlet to vary a configuration of a gas flow path through the inlet; the baffle structure comprising at least two axial offset inlet portions, both of which portions may be located fully (i.e. not partially) within the annular inlet. At least one inlet portion may comprise vanes, dividing an inlet portion into inlet passageways. At least two inlet portions may comprise vanes, dividing the inlet portions into inlet passageways. A configuration of vanes in a first inlet portion may be different from a configuration of vanes in a second inlet portion. A configuration of vanes in a second inlet portion may be the same as a configuration of vanes in a second inlet portion. The baffle structure may be provided in or on an axially moveable sleeve. The sleeve may comprise a solid portion (i.e. not an inlet portion) which may be at least partially locatable within the inlet. The baffle structure may comprise at least three axial offset inlet portions, all three of which portions may be located fully within the annular inlet. A variable geometry turbine may comprise: K a turbine wheel mounted for rotation about a turbine axis witiiin a liousing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; the annular inlet divided into axially adjacent annular portions by at least one annular baffle which is axially spaced from the first and second inlet sidewalls; inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; wherein one of said annular baffle(s) has an axial thickness which is less than the maximum axial thickness of at least one of said Inlet vanes. The vanes are orientated to deflect gas flowing through the annular Inlet towards the direction of rotation of the turbine wheel. While thicker vanes can have certain benefits they can reduce the "throat area" of the annular inlet, i.e. the maximum swallowing capacity of the turbine. The throat area of the inlet can also be reduced by the presence of any other obstructions or constrictions within the annular Inlet. The present invention employs at least one annular baffle provided within the annular inlet so as to divide the inlet Into axially adjacent annular portions. To address the potential problem of the baffle(s) reducing the throat area of the inlet, one (or more when a plurality of baffles are present) of said annular baffle(s) has an axial thickness which is less than the maximum axial thickness of at least one of said inlet vanes. Said one annular baffle preferably has an axial thickness which is less than the maximum axial thickness of all of said inlet vanes. In a preferred embodiment the turbine Incorporates a plurality of baffles and all of said baffles have an axial thickness which Is less than the maximum axial thickness of all of said inlet vanes. The total axial thickness of all of the baffles within the annular Inlet (I.e. the axial thickness of the baffle when only one is present, or the sum of the axial thicknesses of all of the baffles when two or more are present) may be less than around 50 % of the maximum axial thickness of at least one of said inlet vanes. More preferably, the total axial thickness of all of the baffles within the annular Inlet is less than around 25 % of the maximum axial thickness of at least one of said inlet vanes. Still more preferably the total axial thickness of the baffles is less than around 10 % of the maximum axial thickness of at least one of said inlet vanes. It is preferred that the total axial thicl A variable geometry turbine may comprise a turbine wtieel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, inlet vanes extending axially into each of the first and second inlet portions, the inlet vanes defining axially adjacent inlet passages; wherein the configuration of the inlet vanes extending into the first inlet portion differs from the configuration of the inlet vanes extending into the second inlet portion in that the vanes extending into the first inlet portion are circumferentially offset from the vanes extending into the second inlet portion; and wherein the number of vanes in the first inlet portion is more than around 50 % of the number of vanes in the second inlet portion. It is preferred that the number of vanes in the first inlet portion is more than around 75 % of the number of vanes in the second inlet portion, still more preferably around 85 % or more, yet more preferably around 95 to 99 % or more of the number of vanes in the second inlet portion. It is particulariy preferred that the number of vanes in the first inlet portion is substantially similar but less than the number of vanes in the second inlet portion. The first and second inlet portions may be adjacent one another. The vanes in the first and second inlet portions may have substantially the same outer diameter and different inner diameters. Alternatively, the vanes in the first and second inlet portions may have different outer diameters and substantially the same inner diameters. As a further alternative, the respective outer and inner diameters of the vanes in the first and second inlet portions may be different, or may be substantially the same. The circumferential distribution of the vanes extending into the first and/or second inlet portion may be non-uniform. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a ^4 cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into at least three axially offset annular inlet portions by two or more axially spaced annular baffles disposed between the first and second inlet sidewalls; inlet vanes extending axially into at least one of the inlet portions and defining circumferentially adjacent inlet passages; and wherein each of at least two of said baffles extends radially inboard of inlet vanes which extend into at least one of the inlet portions axially adjacent the respective baffle. The at least two baffles which extend radially inboard of inlet vanes may have different internal diameters. According to another aspect of the invention there is provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into at least three axially offset annular inlet portions by two or more axially spaced annular baffles disposed between the first and second inlet sidewalls; inlet vanes extending axially into at least one of the inlet portions and defining circumferentially adjacent inlet passages; and wherein each of at least two of said baffles extends radially inboard of inlet vanes which extend into at least one of the inlet portions axially adjacent the respective baffle, and wherein a distance between an inner diameter of a first baffle of said at least two of said baffles and a trailing edge of a radially innermost vane in one of said annular inlet portions adjacent the first baffle is greater than a distance between an inner diameter of a second baffle of said at least two of said baffles and a trailing edge of a radially innermost vane in one of said annular inlet portions adjacent the second baffle. Said one of said annular inlet portions adjacent the first baffle may be axially displaced from the first baffle in a first direction and wherein said one of said annular inlet portions adjacent the second baffle may be axially displaced from the second baffle in the first direction. At least two baffles which extend radially inboard of inlet vanes in a respective adjacent inlet portions may have different inner diameters. The axial profile formed by the inner diameters of at least two baffles which extend radially inboard of inlet vanes in a respective adjacent inlet portion may generally correspond to an axial profile of a surface that would be swept by the rotation of the turbine wheel. The relative inner diameters of at least three baffles which extend radially inboard of inlet vanes in a respective adjacent inlet portion may generally increase in an axial direction. At least two of the at least two of said baffles may have an inner diameter such that the radial distance relative to the turbine axis between the inner diameter of the baffle and the trailing edge of a radially innemnost vane of an inlet portion adjacent the baffle is more than generally 50%, generally 60%, generally 70%, generally 80%, generally 95% or generally 90% of the radial distance between the trailing edge of said radially innermost vane and the outer diameter of the turbine wheel at the axial position of the baffle. A variable geometry turbine may comprise a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into at least two axially offset annular inlet portions by one or more axially spaced annular baffles disposed between the first and second inlet sidewalls; inlet vanes extending axially into at least one of the inlet portions and defining circumferentially adjacent inlet passages; and wherein at least one of the one or more baffles extends radially inboard of inlet vanes which extend into at least one of the inlet portions axially adjacent the respective baffle, and wherein at least one of said at least one of the one or more baffles has an inner diameter such that the radial distance relative to the turbine axis between the inner diameter of the baffle and the trailing edge of a radially innermost vane of an inlet portion adjacent the baffle is more than generally 50% of the radial distance between the trailing edge of said radially innermost vane and the outer diameter of the turbine wheel at the axial position of the baffle. The radial distance relative to the turbine axis between the inner diameter of the baffle and the trailing edge of a radially innermost vane of an inlet portion adjacent the baffle may be more than generally 60%, generally 70%, generally 80%, generally 90% or generally 95% of the radial distance between the trailing edge of said radially innermost vane and the outer diameter of the turbine wheel at the axial position of the baffle. A variable geometry turbine may comprise:a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; the annular inlet divided into axially adjacent annular portions by at least one annular baffle which is axially spaced from the first and second inlet sidewalls; inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; wherein the configuration of the inlet vanes extending into one of the inlet portions differs from the configuration of the inlet vanes extending into another of the inlet portions and wherein the inner diameter of the sleeve is greater than the outer diameter of the inlet passages. A variable geometry turbine may comprise: a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; the annular inlet divided into axially adjacent annular portions by at least two annular baffles which are axially spaced from the first and second inlet sidewalls; inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; wherein the configuration of the inlet vanes extending into one of the inlet portions differs from the configuration of the inlet vanes extending into another of the inlet portions. 27 In some embodiments it is preferred that the sleeve is movable towards the second inlet sidewall so as to narrow the gas flow path through the inlet, and the gas flow path through the inlet passage that is closer to the second inlet sidewall has a cross-sectional area perpendicular to the direction of gas flow along said path that is smaller than the corresponding cross-sectional area of the gas flow path through the inlet passage that is further away from the second inlet sidewall. The vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a flrst annular portion defines a plurality of first inlet passages having a first total cross-sectional area perpendicular to the direction of gas flow and another array of vanes in a second annular portion, which is axially offset from the first annular portion, deflnes a plurality of second inlet passages having a second larger total cross-sectional area perpendicular to the direction of gas flow. In a preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes defining a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow which decreases progressively between adjacent pairs of annular arrays. In a further preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array defining a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow that is larger or smaller than the total cross-sectional area perpendicular to the direction of gas flow of inlet passages defined by the an-ays of vanes on either side of the intermediate array. The inlet passages having the smallest total cross-sectional area perpendicular to the direction of gas flow may be provided in the annular portion nearest to the second inlet sidewall where the gas flow path through the inlet Is narrowest or substantially closed. Preferably the sum of the minimum circumferential separations between adjacent vanes within the inlet passage closer to the second inlet sidewall is lower than the sum of the minimum circumferential separations between adjacent vanes within the inlet passage further away from the second inlet sidewall. In some embodiments it is preferred that at least one vane in one of said annular portions has a greater maximum circumferential thickness than at least one vane in another of the annular portions. The vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion incorporates a plurality of vanes of a maximum circumferential thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a larger maximum circumferential thickness. It is preferred that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum circumferential thickness which decreases progressively between adjacent pairs of annular arrays. Alternatively, it is preferred that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum circumferential thickness that is larger or smaller than the maximum circumferential thickness of vanes in the arrays of vanes on either side of the intermediate array. Preferably the vanes having larger maximum circumferential thickness are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. It is particularly preferred that vanes having the largest maximum circumferential thickness are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed. It is preferred in the first and/or second aspects of the present invention that at least one vane in one of said annular portions has a greater leading edge thickness than at least one vane in another of the annular portions. Said vanes may be provided in annular arrays within each annular portion. An array of vanes in a first annular portion preferably incorporates a plurality of vanes of a first leading edge thickness and another array of vanes in a second annular portion. which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger leading edge thickness. In a preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a leading edge thickness which decreases progressively between adjacent pairs of annular arrays. In a further embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a leading edge thickness that is larger or smaller than the leading edge thickness of vanes in the arrays of vanes on either side of the intermediate array. It is preferred that vanes having a larger leading thickness are provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. Vanes having the largest leading edge thickness are preferably provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed. It is preferred in the first and/or second aspects of the present invention that at least one vane in one of said annular portions has a greater maximum outer diameter than at least one vane in another of the annular portions. The vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion incorporates a plurality of vanes of a first maximum outer diameter and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger maximum outer diameter. One preferred embodiment provides that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum outer diameter which decreases progressively between adjacent pairs of annular arrays. 30 Another preferred embodiment provides that the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum outer diameter that is larger or smaller than the maximum outer diameter of vanes in the arrays of vanes on either side of the intermediate array. The vanes having a larger maximum outer diameter may be provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. Preferably vanes having the largest maximum outer diameter are provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed. In some embodiments of the present invention it is preferred that at least one vane in one of said annular portions has a greater maximum inner diameter and defines a greater radial clearance between said vane and the turbine wheel than at least one vane in another of the annular portions. Said vanes may be provided in annular arrays within each annular portion. Preferably an array of vanes in a first annular portion incorporates a plurality of vanes of a first maximum inner diameter which define a first radial clearance between said vanes and the turbine wheel and another array of vanes in a second annular portion, which is axially offset from the first annular portion, incorporates a plurality of vanes of a second larger maximum inner diameter which define a second larger radial clearance between said vanes and the turbine wheel. In a preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes having a maximum inner diameter which define a radial clearance between said vanes and the turbine wheel which both decrease progressively between adjacent pairs of annular arrays. In an alternative preferred embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within an intermediate array having a maximum inner diameter which defines a radial clearance between said vanes and the turbine wheel both of which are larger or smaller than the maximum inner diameter of vanes and a 31 radial clearance between said vanes and the turbine wheel in the arrays of vanes on either side of the intermediate array. Vanes having a larger maximum inner diameter and which define a larger radial clearance between said vanes and the turbine wheel are preferably provided in the annular portion(s) nearer to a closed position of the sleeve where the gas flow path through the inlet is narrowest. It is preferred that said vanes with a larger maximum inner diameter and which define a larger radial clearance between said vanes and the turbine also define a larger swirl angle. Vanes having the largest maximum inner diameter and which define the largest radial clearance between said vanes and the turbine wheel are preferably provided in the annular portion nearest to the closed position of the sleeve where the gas flow path through the inlet is narrowest or substantially closed. It is particularly preferred that vanes with the largest maximum inner diameter and which define the largest larger radial clearance between said vanes and the turbine also define the largest swirl angle. In some embodiments of the present invention it is preferred that the number of inlet vanes extending into one of the inlet portions differs from the number of inlet vanes extending into another of the inlet portions. Said inlet portions may be adjacent one another. A variable geometry turbine may comprise: a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; the annular inlet divided into axially adjacent annular portions by at least one annular baffle which is axially spaced from the first and second inlet sidewalls; inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; the sleeve being movable towards the second inlet sidewall so as to narrow the gas flow path through the inlet; wherein the gas flow path through the inlet passage that is closer to the second inlet sidewall has a cross-sectional area perpendicular to the direction of gas flow along 52. said path that is smaller than the corresponding cross-sectional area of the gas flow path through the inlet passage that is further away from the second inlet sidewalk The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel. Gas is deflected along inlet passages defined between neighbouring vanes and adjacent baffles or sidewalls. The "throat area" of the annular inlet, which may be thought of as the maximum gas "swallowing capacity" of the turbine, is the total cross-sectional area perpendicular to the direction of gas flow of all of the inlet passages defined across the annular inlet. One of the parameters which contributes the definition of the throat area is the minimum circumferential separation between circumferentially adjacent vanes within each annular portion. It is thus preferred that the sum of the minimum circumferential distances between adjacent vanes within the inlet passage closer to the second inlet sidewall is lower than the sum of the minimum circumferential distances between adjacent vanes within the inlet passage further away from the second inlet sidewall. By using baffles to divide the annular inlet into two or more annular portions the throat area of each annular portion can be independently defined by the arrangement of the vanes within each annular portion and the axial width of each annular portion. In this way, the throat area of the annular inlet can be varied between the first and second inlet sidewalls. Preferably the gas flow path through the annular inlet is more constricted nearer to the second inlet sidewall, where the gas flow path through the inlet is narrowest or substantially closed, than closer to the first inlet sidewall. The variation in the degree of constriction may be progressive across the axial width of the annular inlet or may vary discontinuously with intermediate annular portions being less constricted than neighbouring annular portions provided that the gas flow path through an inlet passage closer to the second inlet sidewall is more constricted than the gas flow path through an inlet passage that is further away from the second inlet sidewall. Control of the degree of constriction to the gas flow path through the annular inlet by the arrangement of the vanes can be achieved in a number of ways. For example, one or more, or all, of the vanes within one annular portion may have a thickened leading edge, a larger circumferential thickness, or both, as compared to vanes in other annular portions. In a preferred embodiment, vanes with a thicker leading edge are provided in the annular portion(s) nearer to the second inlet sidewall, i.e. the closed position of the sleeve where the gas flow path through the inlet is at its narrowest, since this is where a greater variation in gas incidence angle is to be S3 expected. By way of a further example, a greater number of vanes may be provided in one annular portion than another. For instance, an annular an"ay of fifteen vanes may be included in the same nozzle assembly as an annular array of only eight vanes. Other arrays may have a different number of vanes, greater than fifteen or fewer than eight, or somewhere in between, e.g. twelve. In another example, the swirl angle of vanes in one annular portion may be greater than that in another annular portion. Moreover, the radial extent, outer and/or inner maximum diameter of vanes in one annular portion may be different to that in another annular portion to provide a different degree of constriction in the two annular portions. It will be appreciated that any one or more of the above modifications in vane structure, arrangement or orientation may be employed to achieve the desired variation in throat area across the axial width of the annular iniet. The vanes are preferably provided in annular arrays within each annular portion. An array of vanes in a first annular portion may define a plurality of first inlet passages having a first total cross-sectional area perpendicular to the direction of gas flow and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may define a plurality of second inlet passages having a second larger total cross-sectional area perpendicular to the direction of gas flow. The first inlet passages are preferably provided closer to the second inlet sidewall than the second inlet passages. In one embodiment the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, and the vanes within each annular array of vanes define a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow which decreases progressively between adjacent pairs of annular an-ays. in an alternative embodiment in which the turbine incorporates three or more annular portions each annular portion having a respective annular array of vanes disposed therein, the vanes within an intermediate array define a plurality of inlet passages having a total cross-sectional area perpendicular to the direction of gas flow that is larger or smaller than the total cross-sectional area perpendicular to the direction of gas flow of inlet passages defined by the arrays of vanes on either side of the intermediate array. In a preferred embodiment the inlet passages within the turbine having the smallest total cross-sectional area perpendicular to the direction of gas flow are 3 For certain engine applications (sucli as for exhaust gas recirculation, "EGR") it may be desirable to reduce the turbine efficiency in one or more of the arrays of inlet passageways. For example, it may be desirable to reduce efficiency at relatively open inlet widths in some applications. Such reduced efficiency could for instance be achieved by reducing the radial extent of the vanes (as discussed above), increasing the circumferential width of the vanes, or otherwise configure the vanes to reduce the effective inlet area, i.e. the throat area of the annular inlet. In some embodiments relatively small "splitter vanes" may be located between adjacent pairs of "main" vanes. This arrangement may have the effect of increasing the total number of vanes compared with other embodiments, but the vanes may be provided with a reduced radial extent so that there is a greater radial clearance between the vanes and the turtjine wheel. The splitter vanes may be advantageous in some embodiments to reduce vibration excited in the turbine blades. In some embodiments, the vanes may have a "cut-off' configuration in the region of the trailing edge rather than a full airfoil configuration which can be expected to provide reduced efficiency but which may be useful in some applications. In addition, obstnjctions may be located between adjacent vanes which could further reduce efficiency. The trailing edges of at least some of the vanes extending across a first annular portion of the inlet may lie on a different radius to the trailing edges of at least some of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of all of the vanes extending across a first annular portion of the inlet lie on a radius different to that of the trailing edges of all of the vanes extending across a second annular portion of the inlet. In some embodiments the trailing edges of vanes of one annular portion of the inlet lie on a minimum radius which is different to that of vanes extending across any other annular portion of the inlet. The trailing edges of at least a majority of vanes extending across an annular portion of the inlet may lie on a radius greater than the internal radius of a baffle defining the annular portion. In some embodiments all of the vanes extending across an annular portion may have a trailing edge lying at a radius greater than the internal radius of a baffle defining the annular portion. In some embodiments each annular baffle may have an internal radius smaller than the radius of the leading edge of any vane in the annular inlet. A variable geometry turbine may comprise: a turbine wlieel mounted for rotation about a turbine axis witiiin a iiousing, the liousing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; the annular inlet divided into axially adjacent annular portions by at least one annular baffle which is axially spaced from the first and second inlet sidewalls; inlet vanes extending axially across at least two of said annular portions defined by the or each baffle so as to divide said annular inlet into at least two axially offset inlet passages; wherein at least one vane in one of said annular portions has a greater maximum circumferential thickness than at least one vane in another of the annular portions. The vanes are orientated to deflect gas flowing through the annular inlet towards the direction of rotation of the turbine wheel. A thickened vane can be useful in accommodating greater variation in gas incidence angle without causing flow separation and turbulent flow (efficiency loss). It will be appreciated that thicker vanes reduce the "throat area" of the annular Inlet, i.e. the maximum swallowing capacity of the turbine. As a result, the maximum thickness of a vane or vanes in each annular portion of the inlet or "nozzle section" may be optimized to suit its axial location within the annular inlet and a particular application. By way of example, in a preferred embodiment thicker vanes are provided in the annular portion(s) nearer to the closed position of the sleeve, i.e. where the gas flow path through the inlet is at its narrowest, since this is where a greater variation in gas incidence angle is to be expected. in one preferred embodiment there may be provided at least one annular an^ay of vanes consisting of a relatively small number of vanes configured to define a relatively high swiri angle but which are relatively "thick" and extend to a relatively small internal radius as compared to other arrays of vanes within the same annular inlet so as to provide a relatively small radial clearance around that region of the turbine wheel. With such an arrangement it is easier for an actuator to achieve high resolution control of the cross-sectional flow area because It varies less for a given sleeve movement. The increased swirl may be useful for a vane array positioned to correspond to relatively small inlet widths, which could provide an improvement In efficiency. 37 There is further provided a variable geometry turbine comprising a turbine wheel mounted for rotation about a turbine axis within a housing, the housing defining an annular inlet surrounding the turbine wheel and defined between first and second inlet sidewalls; and a cylindrical sleeve axially movable across the annular inlet to vary the size of a gas flow path through the inlet; wherein the annular inlet is divided into a first annular inlet portion and a second annular inlet portion axially offset from the first inlet portion, inlet vanes extending axially into each of the first and second inlet portions, the inlet vanes defining axially adjacent inlet passages; wherein the configuration of the inlet vanes extending into the first inlet portion differs from the configuration of the inlet vanes extending into the second inlet portion. The inlet vanes may have any suitable configuration, and may for example have a similar general aerofoil configuration to that of known inlet vanes, or they may have any altemative configuration selected to define a particular arrangement and configuration of inlet passages. Since the vanes and inlet baffles together define the configuration and orientation of the inlet passages, a wide variety of different inlet passage configurations can be achieved by appropriate design of the individual nozzle vanes in combination with the inlet baffles. Moreover, the designs can be such that there may be differently configured inlet passages in one annular portion as compared to another annular portion within an annular inlet, or there may be differently configured inlet passages within a single annular portion, or both. For instance, the vanes extending across a first annular portion of the inlet may define a different swirl angle to the vanes extending across a second annular portion of the inlet. The inlet vanes may be provided in annular arrays within each annular portion. The vanes in two or more annular arrays may have different maximum circumferential thicknesses. That is, an array of vanes in a first annular portion may incorporate a plurality of vanes of a first maximum circumferential thickness and another array of vanes in a second annular portion, which is axially offset from the first annular portion, may incorporate a plurality of vanes of a different second circumferential maximum thickness, the first maximum circumferential thickness being larger than the second or vice versa as appropriate. In embodiments incorporating three or more annular portions and therefore three or more annular arrays of vanes, the variation in maximum circumferential thickness of the vanes may decrease progressively between adjacent pairs of annular arrays, or an intermediate array may possess vanes having a maximum circumferential tliicl