accepted from wipo site SECURING AN INTERFACE ELEMENT RAIL OF A ROBOTIC SURGICAL INSTRUMENT INTERFACE FIELD This invention relates to securing a rail supporting a moveable interface element of a robotic surgical instrument interface. BACKGROUND It is known to use robots for assisting and performing surgery. Figure 1 illustrates a typical surgical robot 100 which consists of a base 108, an arm 102, and an instrument 105. The base supports the robot, and is itself attached rigidly to, for example, the operating theatre floor, the operating theatre ceiling or a trolley. The arm extends between the base and the instrument. The arm is articulated by means of multiple flexible joints 103 along its length, which are used to locate the surgical instrument in a desired location relative to the patient. The surgical instrument is attached to the distal end 104 of the robot arm. The surgical instrument penetrates the body of the patient 101 at a port 107 to access the surgical site. At its distal end, the instrument comprises an end effector 106 for engaging in a medical procedure. Figure 2 illustrates a typical surgical instrument 200 for performing robotic laparoscopic surgery. The surgical instrument comprises a base 201 by means of which the surgical instrument connects to the robot arm. A shaft 202 extends between base 201 and articulation 203. Articulation 203 terminates in an end effector 204. In figure 2, a pair of serrated jaws are illustrated as the end effector 204. The articulation 203 permits the end effector 204 to move relative to the shaft 202. It is desirable for at least two degrees of freedom to be provided to the motion of the end effector 204 by means of the articulation. Figure 3 illustrates an example of a known surgical instrument 300 in which end effector 204 is permitted to move relative to shaft 202 by means of pitch joint 301 and two yaw joints 302. Joint 301 enables the end effector 204 to rotate about pitch axis 303. Joints 302 enable each jaw of the end effector 204 to rotate about yaw axis 304. The joints are driven by cables 306, 307 and 308. Pulley 305 is used to direct cables 307 and 308 from their passage over the pitch joint to the yaw joints. Pulley 305 is offset from the central axis of the articulation 203. In a typical laparoscopy operation, a surgeon utilises many instruments, and hence exchanges one instrument for another many times. It is therefore desirable to minimise the time taken and maximise the ease with which one instrument is detached from a robot arm and a different instrument is attached. Additionally, it is desirable to minimise the time taken in setting up the instrument ready for use once it has been attached to the robot arm. As such, the surgical instrument 300 may be attached at its proximal end to the distal end of the robotic arm by an instrument interface. The instrument interface may connect, or engage with, an interface of the robotic arm. Mechanical drive to drive the joints of the instrument (e.g. joints 301 and 302) may be transferred to the instrument from the robotic arm via the robotic arm interface and the instrument interface. SUMMARY According to one aspect of the present disclosure there is provided A robotic surgical instrument, comprising: a shaft; an articulation at a distal end of the shaft for articulating an end effector, the articulation being driveable by a pair of driving elements; and an instrument interface at a proximal end of the shaft, the instrument interface comprising: a chassis; an instrument interface element slideable along a guide bar for driving the pair of driving elements, wherein the pair of driving elements are fast with respect to the interface element so that a displacement of the instrument interface element with respect to the guide bar is transferred to the pair of driving elements; the chassis comprising a support element configured to interface the guide bar along at least a portion of its length; and a securing element for retaining the guide bar against the support element to thereby secure the guide bar to the chassis. The support element may comprise a curved surface that interfaces the guide bar along at least a portion of its length. The support element may be arranged so that each surface normal to the curved surface is transverse to a longitudinal axis of the guide bar. The support element may be a corner feature defining a corner, and the securing element retains the guide bar in the corner to secure the guide bar to the chassis. The corner feature may comprise two surfaces that define the corner. The two surfaces may be planar. The two surfaces may be transverse to each other. The angle between the two surfaces may be less than 180 degrees. The angle between the two surfaces may be greater than or equal to 90 degrees and less than 180 degrees. The angle between the two surfaces may be less than or equal to 90 degrees. The two surfaces may meet to define a join that is parallel to a longitudinal axis of the instrument shaft. The securing element may comprise a shaft portion and a conical-shaped head, the shaft portion being inserted into the chassis to secure the guide bar to the chassis. The shaft portion may be inserted into the chassis parallel to one of the surfaces of the corner feature. The shaft portion may be inserted into the chassis so that the conical-shaped head secures the guide bar against the two surfaces of the corner feature. The shaft portion may be a threaded shaft portion. The securing element may be a countersunk screw or bolt. The securing element may comprise a shaft and a head, the shaft being inserted into the corner feature at an angle to both surfaces of the corner feature so that a longitudinal axis of the shaft is non-parallel to both surfaces. The shaft may be inserted into the corner feature diagonally to the surfaces of the corner feature. The securing element may be a pan-head bolt. The securing element may comprise a retaining element having a first surface shaped to engage the guide bar and being secured to the chassis to thereby secure the guide bar to the chassis. The retaining element may be a block comprising second and third surfaces angled to interface the surfaces of the corner feature. The securing element may be secured to the chassis by one or more bolts or screws. The guide bar may comprise a bore, and the securing element may be a screw or bolt inserted into the support element through the bore to secure the guide bar to the chassis. The instrument interface element may be linearly slideable along the guide bar. The instrument interface element may be linearly slideable along a longitudinal axis of the guide bar parallel to a longitudinal axis of the shaft. The instrument interface may further comprise a second securing element to secure the guide bar to the chassis. The two securing elements may be located at opposing ends of the guide bar. The chassis may comprise a second support element, and the second securing element may retain the guide bar against the second support element to secure the guide bar to the chassis. BRIEF DESCRIPTION OF DRAWINGS The present disclosure will now be described by way of example with reference to the accompanying drawings. In the drawings: Figure 1 illustrates a surgical robot performing a surgical procedure; Figure 2 illustrates a known surgical instrument; Figure 3 illustrates a known arrangement of an articulated end effector of a surgical instrument; Figure 4 illustrates a surgical robot; Figures 5a and 5b illustrate a distal end of a surgical instrument; Figure 6 illustrates a pulley arrangement of the distal end of the surgical instrument of figures 5a and 5b in a variety of non-straight configurations; Figure 7 illustrates arrangements of driving elements in an instrument shaft; Figures 8a and 8b illustrate two views of a surgical instrument including instrument interface; Figures 9a, 9b and 9c illustrate three views of an instrument interface; Figure 10 illustrates a more detailed view of the supporting element and securing elements shown in figure 9. Figure 1 1 illustrates an alternative example of the securing element. Figure 12 illustrates another example of the securing element. Figure 13 illustrates an alternative example of the supporting element. Figure 14 illustrates an alternative example of the securement of a guide bar to the chassis of the instrument interface using a securing element. Figure 15 is a view of the underside of the instrument interface. Figures 16a, 16b and 16c illustrate three views of a drive assembly interface of a robot arm; and Figure 17 illustrates an instrument interface engaged in a drive assembly interface. DETAILED DESCRIPTION The present disclosure is directed to securing a rail supporting a moveable interface element of a robotic surgical instrument interface. A surgical robotic arm can be attached at its distal end to a surgical instrument. The surgical instrument is typically detachable from the robotic arm, e.g. to facilitate the changeover of instruments during a surgical procedure. The surgical instrument may attach to the distal end of the robotic arm via an instrument interface located at the proximal end of the instrument. The instrument interface can engage an interface located at the distal end of the robotic arm. Surgical instruments may comprise an articulation at their distal end for articulating the instrument's end effectors relative to the instrument shaft. The articulation may comprise one or more joints that are mechanically driven. The drive for the joints may be provided by a drive assembly in the robotic arm, for example to save weight in the instrument. In such an arrangement, it is necessary to transfer the drive from the robotic arm to the joints of the instrument articulation. One approach to do this is to transfer the drive via the robotic arm and instrument interfaces. The instrument interface can comprise moveable interface elements (i.e., moveable relative to the instrument interface). Each interface element can be coupled to a joint of the instrument articulation by a pair of driving elements (e.g., cables). Displacement of the instrument interface elements can then cause movement of the driving elements which drives rotation about a joint. The instrument interface elements may be displaced by mechanically engaging the elements with interface elements of the robotic arm interface which are driven by the drive assembly. The instrument interface elements can be slideable along a rail, or bar that guides the displacement of the interface elements when they are driven by the drive assembly. That is, the instrument interface elements can be slideably mounted to the rail, or bar. If the rail is not securely fixed in place, the operation of the surgical instrument may be compromised. The present disclosure describes approaches for securing the rail relative to the instrument interface. Figure 4 illustrates a surgical robot having an arm 400 which extends from a base 401 . The arm comprises a number of rigid limbs 402. The limbs are coupled by revolute joints 403. The most proximal limb 402a is coupled to the base by joint 403a. It and the other limbs are coupled in series by further ones of the joints 403. A wrist 404 is made up of four individual revolute joints. The wrist 404 couples one limb (402b) to the most distal limb (402c) of the arm. The most distal limb 402c carries an attachment 405 for a surgical instrument 406. Each joint 403 of the arm has one or more motors 407 which can be operated to cause rotational motion at the respective joint, and one or more position and/or torque sensors 408 which provide information regarding the current configuration and/or load at that joint. The motors may be arranged proximally of the joints whose motion they drive, so as to improve weight distribution. For clarity, only some of the motors and sensors are shown in figure 4. The arm may be generally as described in our co-pending patent application PCT/GB2014/053523. The arm terminates in an attachment 405 for interfacing with the instrument 406. The instrument 406 may take the form described with respect to figure 2. The attachment 405 comprises a drive assembly for driving articulation of the instrument, and a drive assembly interface for engaging an instrument interface of the instrument 406. Movable interface elements of the drive assembly interface mechanically engage corresponding movable interface elements of the instrument interface in order to transfer drive from the robot arm to the instrument. One instrument may be exchanged for another several times during a typical operation. Thus, the instrument is attachable and detachable from the robot arm during the operation. Features of the drive assembly interface and the instrument interface may aid their alignment when brought into engagement with each other, so as to reduce the accuracy with which they need to be aligned by the user. The instrument 406 comprises an end effector for performing an operation. The end effector may take any suitable form. For example, the end effector may be smooth jaws, serrated jaws, a gripper, a pair of shears, a needle for suturing, a camera, a laser, a knife, a stapler, a cauteriser, a suctioner. As described with respect to figure 2, the instrument comprises an articulation between the instrument shaft and the end effector. The articulation may comprise one or more joints which permit the end effector to move relative to the shaft of the instrument. The one or more joints in the articulation are actuated by driving elements, such as cables. These driving elements are secured at the other end of the instrument shaft to the interface elements of the instrument interface. Thus, the robot arm transfers drive to the end effector as follows: movement of a drive assembly interface element moves an instrument interface element which moves a driving element which moves a joint of the articulation which moves the end effector. Controllers for the motors, torque sensors and encoders are distributed with the robot arm. The controllers are connected via a communication bus to control unit 409. A control unit 409 comprises a processor 410 and a memory 41 1 . Memory 41 1 stores in a non-transient way software that is executable by the processor to control the operation of the motors 407 to cause the arm 400 to operate in the manner described herein. In particular, the software can control the processor 410 to cause the motors (for example via distributed controllers) to drive in dependence on inputs from the sensors 408 and from a surgeon command interface 412. The control unit 409 is coupled to the motors 407 for driving them in accordance with outputs generated by execution of the software. The control unit 409 is coupled to the sensors 408 for receiving sensed input from the sensors, and to the command interface 412 for receiving input from it. The respective couplings may, for example, each be electrical or optical cables, or may be provided by a wireless connection. The command interface 412 comprises one or more input devices whereby a user can request motion of the end effector in a desired way. The input devices could, for example, be manually operable mechanical input devices such as control handles or joysticks, or contactless input devices such as optical gesture sensors. The software stored in memory 41 1 is configured to respond to those inputs and cause the joints of the arm and instrument to move accordingly, in compliance with a pre-determined control strategy. The control strategy may include safety features which moderate the motion of the arm and instrument in response to command inputs. Thus, in summary, a surgeon at the command interface 412 can control the instrument 406 to move in such a way as to perform a desired surgical procedure. The control unit 409 and/or the command interface 412 may be remote from the arm 400. Figures 5a and 5b illustrate opposing views of the distal end of an example surgical instrument. In figures 5a and 5b, the end effector 501 comprises a pair of end effector elements 502, 503, which in this example are depicted as a pair of opposing serrated jaws. It will be understood that this is for illustrative purposes only. The end effector may take any suitably form, such as those described above. The end effector 501 is connected to the instrument shaft 504 by articulation 505. Articulation 505 comprises joints which permit the end effector 501 to move relative to the shaft 504. In this example, the articulation 505 comprises three joints. A first joint 506 permits the end effector 501 to rotate about a first axis 510. The first axis 510 is transverse to the longitudinal axis of the shaft 51 1 . The first joint 506 is arranged so that the shaft 504 terminates at its distal end in the joint 506. A second joint 507 permits the first end effector element 502 to rotate about a second axis 512. The second axis 512 is transverse to the first axis 510. A third joint 513 permits the second end effector element 503 to rotate about the second axis 512. The first end effector element 502 and the second end effector element 503 may be independently rotatable about the second axis 512 by the second and third joints. The end effector elements may be rotated in the same direction or different directions by the second and third joints. The first end effector element 502 may be rotated about the second axis, whilst the second end effector element 503 is not rotated about the second axis. The second end effector element 503 may be rotated about the second axis, whilst the first end effector element 502 is not rotated about the second axis. Figures 5a and 5b depict a straight configuration of the surgical instrument in which the end effector is aligned with the shaft 504. In this orientation, the longitudinal axis of the shaft 51 1 is coincident with the longitudinal axis of the articulation and the longitudinal axis of the end effector. Articulation of the first, second and third joints enables the end effector to take a range of attitudes relative to the shaft. The articulation 505 comprises a supporting body 509. At one end, the supporting body 509 is connected to the shaft 504 by the first joint 506. At its other end, the supporting body 509 is connected to the end effector 501 by second joint 507 and third joint 513. Thus, first joint 506 permits the supporting body 509 to rotate relative to the shaft 504 about the first axis 510; and the second joint 507 and third joint 513 permit the end effector elements 502, 503 to rotate relative to the supporting body 509 about the second axis 512. In the figures, the second joint 507 and third joint 513 both permit rotation about the same axis 512. However, the second and third joints may alternatively permit rotation of the end effector elements about different axes. The axis of rotation of one of the end effector elements may be offset in the longitudinal direction of the shaft 504 from the axis of rotation of the other end effector element. The axis of rotation of one of the end effector elements may be offset in a direction transverse to the longitudinal direction of the shaft 504 from the axis of rotation of the other end effector element. The axis of rotation of one of the end effector elements may not be parallel to the axis of rotation of the other end effector element. The axes of rotation of the end effector elements 502, 503 may be offset in the longitudinal direction of the shaft and/or offset in a direction perpendicular to the longitudinal direction of the shaft and/or angled with respect to each other. This may be desirable as a result of the end effector elements being asymmetric. For example, in an electrosurgical element, a first end effector element may be powered and a second end effector element not powered and insulated from the first end effector element. To aid this, the axes of rotation of the two end effector elements may be offset in the direction perpendicular to the longitudinal direction of the shaft. In another example, a first end effector element may be a blade and a second end effector element a flat cutting surface. To aid use of the blade, the axes of rotation of the two end effector elements may be angled to one another. The joints of the articulation 505 are driven by driving elements. The driving elements are elongate elements which extend from the joints in the articulation through the shaft 504 to the instrument interface. Each driving element may be capable of being flexed laterally to its main extent at least in those regions where it engages the internal components of the articulation and instrument interface. In other words, each driving element can be flexed transverse to its longitudinal axis in the specified regions. This flexibility enables the driving elements to wrap around the internal structure of the instrument, such as the joints and pulleys. The driving elements may be wholly flexible transverse to their longitudinal axes. The driving elements may be inflexible along their main extents. The driving elements may resist compression and tension forces applied along their length. In other words, the driving elements may resist compression and tension forces acting in the direction of their longitudinal axes. The driving elements may have a high modulus. The driving elements may remain taut in operation; they may be not permitted to become slack. Thus, the driving elements are able to transfer drive from the instrument interface to the joints. The driving elements may be cables, for example. Each joint may be driven by a pair of driving elements. Referring to figures 5a and 5b, the first joint 506 is driven by a first pair of driving elements A1 ,A2. The second joint 507 is driven by a second pair of driving elements B1 ,B2. The third joint is driven by a third pair of driving elements C1 ,C2. Each joint of instrument 501 is therefore driven by its own pair of driving elements. In other words, each joint is driven by a dedicated pair of driving elements. The joints may be independently driven. A pair of driving elements may be constructed as a single piece as shown for the third pair of driving elements in figures 5a and 5b. In this case, the single piece is secured to the joint at one point. For example, the third pair of driving elements C1 ,C2 comprises a ball feature 520 which is secured to the third joint 513. This ensures that when the pair of driving elements is driven, the drive is transferred to motion of the joint about its axis. Alternatively, a pair of driving elements may be constructed as two pieces. In this case, each separate piece is secured to the joint. Figure 6 illustrates the distal end of the surgical instrument in five different configurations. Configuration (c) is the straight configuration previously mentioned, in which the end effector is aligned with the instrument shaft. In configurations (a), (b), (d) and (e), rotation about the first joint has occurred relative to configuration (c). In configurations (a), (b), (d) and (e), no rotation about either the second or third joint has occurred relative to configuration (c). Starting from configuration (c), the driving element A2 (not shown) is pulled in order to cause the rotation about the first axis 510 leading to the arrangement of configuration (b). The driving element A2 is further pulled to cause further rotation about the first axis 510 to lead to the arrangement of configuration (a). Starting from configuration (c), the driving element A1 (not shown) is pulled in order to cause rotation about the first axis 510 in an opposing direction to that in configurations (a) and (b), thereby leading to the arrangement of configuration (d). The driving element A1 is further pulled to cause further rotation about the first axis 510 to lead to the arrangement of configuration (e). Rotation of the end effector 501 about the first axis 510 is bounded by the maximum travel of the first pair of driving elements A1 ,A2 about the first joint 506. Configuration (a) shows the end effector 501 at maximum rotation about the first axis 510 in one direction, and configuration (e) shows the end effector 501 at maximum rotation about the first axis 510 in the opposing direction. The maximum rotation angle relative to the longitudinal axis of the shaft 51 1 in both configurations is the angle φ. The first, second and third pairs of driving elements A1 ,A2, B1 ,B2, C1 ,C2 extend through the instrument shaft from the distal end of the shaft 504 connected to the articulation to the proximal end of the shaft connected to a drive mechanism of the instrument interface. Figures 8a and 8b illustrate two views of the first, second and third pairs of driving elements extending from the described articulation to an exemplary instrument interface 801. In an exemplary implementation, the second and third pairs of driving elements overlap in the shaft so as to emerge from the proximal end of the shaft in a different arrangement to that at which they are in at the distal end of the shaft. Figures 7a and 7b illustrates cross-sections of the shaft depicting the positions of the driving elements according an exemplary implementation. Figure 7a shows a cross-section of the shaft at the distal end of the shaft illustrating the positions of the driving elements. The driving elements A1 and A2 are at opposing sides of the shaft after having left the first joint 506. The driving elements C1 and B2 are adjacent each other on an opposing side of the shaft to the driving elements B1 and C2 which are also adjacent each other. The driving elements C1 and B2 are offset from the driving elements B1 and C2 about an axis 701 which is transverse to the axis 702 connecting driving elements A1 and A2. Figure 7b shows a cross-section of the shaft at the proximal end of the shaft illustrating the positions of the driving elements. In other words, configuration (b) shows the positions of the driving elements as they are about to exit the shaft into the instrument interface. The first pair of driving elements A1 and A2 are on opposing sides of the shaft in a similar arrangement to their arrangement in figure 7a. The first pair of driving elements may be closer together, by virtue of them having moved slightly towards each other over the course of their extent through the shaft. In figure 7b, driving element B1 is located on an opposing side of the shaft to its location in figure 7a. In figure 7b, driving element C1 is located on an opposing side of the shaft to its location in figure 7a. To achieve this, driving element B1 and driving element C1 have not extended down the shaft parallel to the longitudinal axis of the shaft 51 1 . Instead, driving element B1 and driving element C1 have overlapped each other during their extent in the shaft. This overlapping occurs without the driving elements B1 and C1 clashing because of their offset positions in figure 7a. Driving element B2 has moved a little in the shaft, but remained on the same side of the shaft as in figure 7a, so as to emerge at the proximal end of the shaft adjacent to driving element B1 . Driving element C2 has moved a little in the shaft, but remained on the same side of the shaft as in figure 7a, so as to emerge at the proximal end of the shaft adjacent to driving element C1 . The driving elements A1 , A2, B1 , B2, C1 and C2 emerge at the proximal end of the shaft in a configuration which enables them to engage directly with components of the instrument interface. Referring back to figures 8a and 8b, the instrument interface is relatively flat. The instrument interface extends mostly in a central plane viewed head on in figure 8a. The instrument shaft 504 is rigidly attached to the instrument interface 801. The instrument shaft 504 does not rotate or otherwise move relative to the instrument interface 801 . The second axis 512 about which the end effector elements 502, 503 rotate is in this example perpendicular to the central plane of the instrument interface. This is the case in the straight configuration of the instrument shown in figures 8a and 8b. Thus, in the straight configuration of the instrument, the jaws of the end effector are moveable in the central plane of the instrument interface. A driving element may be a uniform component having the same shape and size along its length and constructed of the same material along its length. Alternatively, the driving element may be composed of different portions. In one example, the portion of the driving element which engages components of the instrument interface (such as pulleys and interface elements) is flexible. Similarly, the portion of the driving element which engages components of the distal end of the surgical instrument (such as the pulleys and joints in the articulation) is flexible. Between these two flexible portions are spokes 802 illustrated in figures 8a and 8b. Thus, each pair of driving elements comprises two spokes and two flexible portions. Each pair of driving elements forms a loop. The loop comprises alternating spokes and flexible portions. The two spokes are predominantly or wholly enclosed in the instrument shaft. A distal flexible portion terminates at one end in the distal end of one of the spokes, and at the other end in the distal end of the other spoke. The distal flexible portion engages components of the articulation. A proximal flexible portion terminates at one end in the proximal end of one of the spokes, and at the other end in the proximal end of the other spoke. The proximal flexible portion engages components of the instrument interface. The spokes are stiffer than the flexible portions. Suitably, the spokes are rigid. The spokes may be hollow. Typically, the spokes have a larger diameter than the flexible portions. Thus, the flexible portions may be cables, and the spokes hollow tubes. The flexible portions may terminate where they meet the spokes. Alternatively, the spokes may encapsulate the material of the flexible portions. For example, the spokes may be rigid sheaths which cover flexible cables. The spokes are stiffer than the flexible portions. Thus, by forming a pair of driving elements from spokes as well as flexible portions, the likelihood of the driving element stretching is reduced. For this reason, the proportion of each driving element which is a spoke is preferably maximised whilst ensuring that the spoke does not come into contact with components of the articulation or the instrument interface, and also that adjacent driving elements do not collide. The spokes are stronger than the flexible portions, and hence more resilient to compression and tension forces applied in any direction than the flexible portions. Thus, by incorporating the spokes, the driving element as a whole is stiffer and less likely to stretch. Thus, the lifetime of the driving element before it needs re-tensioning or replacing is extended. Mechanical drive from the robotic arm is transferred to the surgical instrument to articulate the joints of the instrument articulation via the instrument interface 801 and drive assembly interface. To drive a joint of the instrument articulation, an interface element of the drive assembly interface is moved, which moves a mechanically engaged interface element of the instrument interface. Movement of the instrument interface element moves a driving element, which drives a joint of the articulation. The mechanism by which the mechanical drive is transferred will be explained in more detail below with reference to figures 9a, 9b and 9c. Figures 9a, 9b and 9c illustrate three more detailed views of the instrument interface 801 . The instrument interface 801 comprises a chassis 900 that supports a drive mechanism (denoted generally at 901 ) for driving the joints of the instrument articulation. The drive mechanism comprises an arrangement of drive elements and pulleys which transfer drive provided by the robotic arm to the joints, as will be described in more detail below. As shown in Figures 9b and 9c, the instrument interface comprises three interface elements 905, 906 and 907. The instrument interface elements form part of the instrument interface drive mechanism. The first instrument interface element 905 engages the first pair of driving elements A1 ,A2. A second instrument interface element 906 engages the second pair of driving elements B1 , B2. A third instrument interface element 907 engages the third pair of driving elements C1 ,C2. Each driving element is secured to its associated instrument interface element. In other words, each driving element is fast with its associated instrument interface element. Thus, in the examples illustrated in figures 9a, 9b and 9c, each pair of driving elements engages a single instrument interface element in the instrument interface 801 . Each driving element engages an instrument interface element in the instrument interface. In other words, each driving element engages its own instrument interface element. A single instrument interface element drives a pair of driving elements. Each driving element is driven independently by a single instrument interface. In alternative arrangements, there may be a compound driving motion in which more than one instrument interface element drives a single driving element, a single instrument interface element drives more than one pair of driving elements, or a plurality of instrument interface elements collectively drive a plurality of driving elements. The instrument interface elements 905, 906 and 907 are dispersed across the width of the instrument interface as shown in figure 9b. In the arrangement depicted in figure 9b, one instrument interface element 905 is within the internal portion 950 of the instrument interface. Specifically, the part of the instrument interface element 905 which engages the driving element is within the internal portion 950 of the instrument interface. The instrument interface element 905 as a whole may be substantially within the internal portion 950 of the instrument interface, as shown in figure 9b. The instrument interface element 905 as a whole may be wholly within the internal portion 950 of the instrument interface. The instrument interface element 905 is in this example aligned with the longitudinal axis 51 1 of the shaft 504. In an exemplary arrangement, only one instrument interface element is located within the internal portion of the instrument interface. The remainder of the instrument interface elements 906, 907 are within the external portion of the instrument interface. These other instrument interface elements 906, 907 are located on either side of the aligned instrument interface element 905. Specifically, the other instrument interface elements 906, 907 are located on either side of the aligned instrument interface element 905 in a direction perpendicular to the longitudinal axis of the shaft 51 1 . The instrument interface elements 906 and 907 are not aligned with the longitudinal axis 51 1 of the shaft 504. Instrument interface element 905 engages a first pair of driving elements A1 , A2. As can be seen in figure 9a, between the proximal end of the shaft and the instrument interface element 905, the pair of driving elements A1 , A2 lie wholly within the internal portion 950. Between the proximal end of the shaft and the instrument interface element 905, the pair of driving elements A1 , A2 lie wholly parallel to the longitudinal axis of the shaft 51 1 . In the arrangement shown, there are no intervening pulleys or other structures in the instrument interface around which the pair of driving elements A1 , A2 is constrained to move between the proximal end of the shaft and the instrument interface element 1905. Only instrument interface element 905 engages its pair of driving elements A1 , A2 in the internal portion 950 of the instrument interface in this arrangement. Instrument interface element 906 engages a second pair of driving elements B1 , B2. The instrument interface element 906 engages the second pair of driving elements B1 , B2 in the external portion of the instrument interface. Instrument interface element 907 engages a third pair of driving elements C1 , C2. The instrument interface element 907 engages the third pair of driving elements C1 , C2 in the external portion of the instrument interface. A pulley arrangement is used to shift the driving elements over to engage with the instrument interface elements which are in the external portion. Each pair of driving elements engages a first pair of pulleys to shift it over from the proximal end of the shaft 504 to its respective instrument interface element, and a second pair of pulleys to shift it back from alignment with the instrument interface element to alignment with the shaft 504. In the arrangement shown, the second pair of driving elements B1 , B2 emerges from the proximal end of the shaft in a direction aligned with the shaft. The driving elements B1 ,B2 do not run exactly parallel to the longitudinal axis 51 1 of the shaft 504 as a result of the direction changes described with respect to figure 7. The second pair of driving elements B1 , B2 is then constrained to move around pulley pair 908 and 909 to shift it from where it emerges from the shaft 504 to engagement with the second instrument interface element 906. The second pair of driving elements B1 , B2 emerges from the pulley pair 908 and 909 in a direction parallel to and offset from the direction that the second pair of driving elements B1 , B2 emerges from the proximal end of the shaft. The second pair of driving elements B1 ,B2 is constrained to move around pulley pair 910 and 91 1 to shift it from alignment with the second instrument interface element 906 to alignment with the shaft 504. In the arrangement shown, the third pair of driving elements C1 , C2 emerges from the proximal end of the shaft in a direction aligned with the shaft. The driving elements C1 ,C2 do not run exactly parallel to the longitudinal axis 51 1 of the shaft 504 as a result of the direction changes described with respect to figure 7. The third pair of driving elements C1 ,C2 is then constrained to move around pulley pair 912 and 913 to shift it from where it emerges from the shaft 504 to engagement with the third instrument interface element 907. The third pair of driving elements C1 , C2 emerges from the pulley pair 912 and 913 in a direction parallel to and offset from the direction that the third pair of driving elements C1 , C2 emerges from the proximal end of the shaft. The third pair of driving elements C1 ,C2 is constrained to move around pulley pair 914 and 915 to shift it from alignment with the third instrument interface element 907 to alignment with the shaft 504. Thus, to summarise, in the arrangement shown in figures 9a, 9b and 9c, pair of driving elements A1 , A2 engage with the first instrument interface element 905. Pair of driving elements A1 , A2 drive rotation of the articulation, and hence the end effector, about the first axis 510 (see figure 5a). The pair of driving elements B1 , B2 engage with the second instrument interface 906. Driving elements B1 ,B2 drive rotation of the second joint 507. The pair of driving elements C1 ,C2 engage with the third instrument interface 907. Driving elements C1 ,C2 drive rotation of the third joint 513. Thus, each joint of the instrument articulation is driven by a respective pair of driving elements, and each pair of driving elements is in turn driven by a respective instrument interface element. Each instrument interface element is displaceable within the instrument interface 801 to drive its respective pair of driving elements. Since each instrument interface element is secured to a corresponding pair of driving elements, a displacement of the instrument interface element is transferred to a displacement of the pair of driving elements. Each instrument interface element may be displaceable along the same line as the line of the pair of driving elements that it is secured to. Each instrument interface element engages with a corresponding drive assembly interface element of the robot arm. Thus, displacement of the instrument interface element is driven by the robot arm. In this way, the robot arm drives the pairs of driving elements (and hence the joints of the instrument articulation). Each instrument interface element 905, 906 and 907 is linearly displaceable within the instrument interface 801 . The interface elements may be displaceable along a displacement axis parallel to the longitudinal axis of the shaft 51 1 . Each instrument interface element is mounted to a rail to support, or constrain, or guide, the motion of the interface element within the instrument interface. The rail may therefore be referred to as a guide bar. The rail/guide bar may be linear. As shown most clearly in figures 9b and 9c, the first instrument interface element 905 is mounted to rail 928; the second instrument interface element 906 is mounted to rail 929; and the third instrument interface element 907 is mounted to rail 930. The interface elements are slideably mounted to the rails to permit relative linear motion between the rail and the interface elements. That is, each interface element 905, 906, 907 is slideable along its respective rail 928, 929, 930. The rails are fast with respect to the chassis 900, and thus the interface elements are slideable relative to the chassis. Each instrument interface element can be displaced over a displacement range between a minimum displacement position and a maximum displacement position. For example, the minimum and maximum displacement positions may be determined by the ends of the rail along which the instrument interface element slides. The minimum and maximum displacement positions are labelled 931 and 932 on figures 9b for the second and third instrument interface elements 906 and 907. The minimum and maximum displacement positions are labelled 931 and 943 on figure 9b for the first instrument interface element 905. The first instrument interface element is linearly displaceable through a maximum distance di minus the length of the first instrument interface element in the direction x. The second instrument interface element is linearly displaceable through a maximum distance d2 minus the length of the second instrument interface element in the direction x. The third instrument interface element is linearly displaceable through a maximum distance d3 minus the length of the third instrument interface element in the direction x. Here, di