CONDENSATE TRAP FIELD OF THE INVENTION This invention. relates to condensate traps. BACKGROUND OF THE INVENTION Condensate traps are coramonly employed to remove condensed w'ater from steam utilizing plant and equipment, in which context they are normally referred to as steam traps. Many different designs of steam traps have been developed to suit a variety of circumstances. The majority of traps involve a self-actuating mechanism which detects the presence of condensate in the trap, and when necessary opens to allow the condensate to drain. These traps have moving parts and consequently are prone to suffering mechanical failure. An alternative form of trap is a fixed orifice trap. These are generally reliable as they have no moving parts, and in the simplest form comprise an aperture through which condensate is discharged. Flash steam produced as the pressure drops during flow through the aperture acts to reduce the amount of steam that escapes through the aperture. One problem encountered with fixed orifice traps is that the aperture may become blocked. A further problem is that it is difficult to select an orifice size which matches the condensate load exactly, so that condensate is discharged while preventing live steam from leaking through the aperture. In reality fixed orifice traps tend to be oversized to ensure that condensate is effectively removed while the consequent loss of steam, when little oi- no condensate is present, is accepted. SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided a condensate trap comprising a vortex chamber, an inlet being provided for admitting fluid into the chamber in a manner to promote a rotational flow of the fluid in the chamber about a longitudinal axis of the chamber, and an escape aperture being provided at an axial end of the chamber. Preferably at least a portion of the side wall nas a substantially circular cross section and may be cylindrical, In one embodiment, the cylindrical portion adjoins the wider diameter end of a frusto conical portion. The escape aperture may be provided at the narrower end of the frusto conical portion, for example on the longitudinal axis of the chamber. The chamber may have a circular transverse end wall, the escape aperture being provided in the centre of the end wall. In a practicai embodiment of a steam trap in accordance with the present invention, the vortex chamber may be provided in a control element supported by a body provided with inlet and outlet passages which communicate respectively with the inlet to the chamber and the escape aperture. The body may be provided with means for connecting the inlet and outlet passages to pipework of a steam utilisation plant. The control element and the body may abut each other at respective contact surfaces, which are preferably flat and circular. Ports are preferably then provided at the contact surfaces to provide communication between the inlet and outlet passages in the body and the respective inlet and escape aperture in the control element. The control element may be engageable with the body at the contact surfaces in any one of a plurality of different rotational positions, for example if the contact surfaces are circular as mentioned above. The port communicating with the escape aperture may serve as the centre of rotation between the different rotational positions. To ensure communication between the inlet passage and the inlet in all possible rotational positions, the port communicating with the inlet may take the form of a circular groove centred on the port communicating with the escape aperture. The inlet may be one of a plurality of inlets, for example three inlets, which are equally distributed around the chamber and which are directed tangentially of the chamber to induce the vortex. The control element may be secured to the body by means of a cap, which clamps the control element to the contact surface of the body. The chamber may open at a face of the control element opposite the contact surface, in which case the cap preferably closes the chamber. A second inlet may be provided, which directs the fluid towards the central longitudinal axis of the chamber. The second inlet may be provided at the same longitudinal portion along the vortex chamber as the first inlet. Switch means may be provided to select either the first or second inlet to introduce the fluid into the chamber. The switch means may be responsive to temperatura sensing means, such as a bimetallic strip, which senses the temperature of the fluid upstream of the trap. The escape aperture may have any suitable diameter, depending on the required discharge rate of condensate. In most circumstances, it is envisaged that the escape aperture diameter will făli in the range l mm to 40 mm, although in many embodiments the escape aperture diameter will be less than 30 mm, and possibly less than 10 mm. For example, the diameter of the escape aperture may be 5 mm. According to a second aspect of the present invention, there is provided a method of reducing a flow of steam through an escape aperture of a steam trap, comprising the steps of: directing a fluid comprising a mixture of steam and condensate into a chamber in a direction so as to create a vortex within the chamber, and providing the escape aperture directly downstream of a low pressure location within the vortex. BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:- Figure l is a cross section view of a fixed orifice steam trap in accordance with a first embodiment of the present invention; Figure 2 is a plan view of the fixed orifice steam trap of Figure l as viewed from the line II-II in Figure l; Figure 3 is a cross section view of a fixed orifice steam trap in accordance with a second embodiment of the present invention; Figure 4 is a plan view of the fixed orifice steam trap of Figure 3 as viewed from the line IV-IV in Figure 3; Figure 5 is a sectioned view of another embodiment of a steam trap; Figure 6 is a plan view of the steam trap o'f Figure; and Figure 7 is a plan view of a component of the steam trap of Figures 5 and 6. DETAILED DESCRIPTIQN With reference to Figure l, the steam trap l comprises an upper flange 2, a lower flange 6, and a main body 4 secured between the upper and lower flanges 2,6. The body 4 defines a vortex chamber 8, having an upper portion 10 and a lower portion 12. The upper portion 10 of the vortex chamber is cylindrical and closed off at its upper edge by the upper flange 2. The lower portion is a conical frustum, continuing from the cylindrical wall of the upper portion 10 and tapering to a smaller diameter at its flat base 14. An escape aperture 16 is provided in the centre of the base 14 on the central axis 30 of the trap l, leading to a conduit 18 extending vertically downwardly from the aperture 16. The escape aperture 16 is approximately 5 mm diameter in this example. This communicates at its lower opening to the surrounding environment or to a condensate return pipe (not shown), via a circular opening 5 in the lower flange 6. The conduit 18 has a length which is greater than the diameter of the escape aperture, for example greater than twice the diameter of the escape aperture. In the embodiment shown, the length of the conduit 18 is 12 mm. A substantxally cylindrical inlet passage 20 is provided in the main body 4, and opens into the wall of the chamber 8 at the lower region of the upper portion 10 of the chamber 8. Referring now to Figure 2, the outermost edge 22 of the cylindrical inlet passage 20 continues tangentially from the cylindrical wall of the chamber 8. The innermost edge 24 of the inlet passage 20 is offset from the central axis 30 of the chamber 8 towards the outer edge 22. At the end of the inlet 20 away from the chamber 8, a connector 26 is provided to connect the inlet 20 to a source of steam and condensate. In use, steam and condensate is introduced tangentially into the chamber 8 through the inlet 20. With the inlet 20 oriented as shown in Figure 2, the steam and condensate flows around the chamber wall 8 in an anticlockwise direction, creating a vortex. The central axis of the vortex lies on or close to the central axis 30 of the chamber 8. The vortex naturally generates a low pressure at its centre. As the escape aperture 14 is located on the central axis 30, the vortex thus provides a low pressure region directly upstream of the aperture 16. This reduces the discharge rate through the escape aperture, and accordingly a larger escape aperture can be used, reducing the likelihood of the aperture becoming blocked. Additionally, the seif regulatory raechanisms of the vortex provide increased condensate discharge in relation to the discharge of steam leaking from the aperture, as the trap exhibits the following characteristics: During plant start up, cold condensate is built up throughout the system and the condensate load on the trap is at its highest at this time. As the water is cold, little or no flash steam will be created, as the water is typically not near the steam saturation temperature and so will not flash to steam at the pressures encountered, either at the low pressure centre of the vortex, or at pressure of the outside environment. Therefore the cold water will flow freely through the escape aperture 16. However, as the temperature at the steam trap increases, the water will eventually reach the saturation temperature for the pressure prevailing at the centre of the vortex. At this point vapour or flash steam will begin to form, and it is this flash steam that will be expelled through the escape aperture 16. This has the effect of reducing the discharge through the escape aperture 16, as the density of the flash steam is much lower than that of water. The low pressure created at the centre of the vortex also reduces the pressure drop from upstream to downstream of the escape aperture 16. Therefore this will also reduce the discharge rate of the aperture 16 as an effect additional to the creation of flash steam as described above . The f low in the trap is in practice very complex. In accordance with the present invention the vortex chamber creates an area of low pressure upstream of the escape aperture. As is well known in the art, in a vortex pressure energy is converted to kinetic energy. From the conservat ion of energy (Bernoulli) equation, as the velocity increases the pressure f alls . Therefore, low pressure results in low density at any given point, in this case the centre of the vortex. By creat ing this condition, as steam reaches the escape orifice, the mass discharge rate is reduced in accordance with the f low equation for a single phase fluid: Q = Cd X (ftd / 4) X (p x Apc) where Q = flow rate C