The present invention relates to electrical machines and more particularly to so-called Λswitched reluctance electrical machines', which include a rotor core arranged to rotate within a stator housing. Utilisation of so-called Λswitched reluctance electrical machines' is advantageous due to their relatively simple construction, which in turn facilitates operation at higher rotational speeds with consequent high power density, particularly when such electrical machines are utilised for engine driven electricity generators. However, a problem relates to the necessary castellated outer surface resultant from the salient-pole design. It would be appreciated that upon rotation this castellated surface and the cavities between each castellation creates excessive aerodynamic losses (windage) and overheating at high speeds within the stator core. In order to avoid the windage and overheating problems, attempts have previously been made to create a substantially smooth external rotor core surface. This smooth external surface can be achieved either by providing a close fitting cylindrical sleeve about the core or by locating filling members in the cavities between the salient poles of the rotor. The closely fitting sleeve approach inherently reduces magnetic efficiency and magnetic power, and contrary to one of the principal attractions of ^switched reluctance machines' increases the size of that machine for a given output. However, the approach of incorporating filling members into the cavities between poles encounters a significant problem with respect to rotational and other forces. In such circumstances, it is desirable to improve the aerodynamic performance of a switched reluctance electrical machine whilst maintaining the advantages of high power density in terms of rotational speed, etc. In accordance with the present invention there is provided an electrical machine comprising a rotor core with surface cavities and fillet members for each cavity, each fillet member being pre-stressed for resilient location within its cavity despite rotation and/or other loading presented in use to the rotor core. Alternatively, in accordance with the present invention, there is provided a fillet member for a cavity of an electrical machine rotor core, the member configured for a cavity to be pre-stressed when located within that cavity to resist rotational and/or other loadings which may displace the member from the cavity. Preferably, the fillet member is shaped to provide pre- stressing. Typically, that shaping is through a curved nature when in a relaxed state. Possibly, the fillet member is deformed in situ in order to provide pre-stressing. Additionally, the fillet member may include a specific stressing element in order to facilitate pre-stressing of that fillet member within its cavity. Possibly, specific portions of the fillet member are stressed in order to provide the pre-stressing of the fillet member in use. Typically, the pre-stressing creates radial loads across its cavity. Typically, the fillet member engages annular features of the cavity and/or core. Generally, these annular features incorporate an overhanging structure in order to locate the fillet member. Possibly, the ends of the fillet member are tapered to engage the cavity. Generally, the cavities are formed as castellations in the surface of the rotor core. Normally, the rotor core incorporates an end cap in order to retain each fillet member in use within its cavity. Typically, the fillet members are configured in order to provide substantially uniform load distribution through and across the fillet member when pre-stressed within its cavity. Possibly, the fillet members are hollow. Advantageously, fillet members are slid into position within their respective cavities. Alternatively, the fillet members are snap-fit into their respective cavities. Possibly, pre-stressing within the fillet member can be initiated or increased in situ within the fillet member through appropriate activation. Possibly, activation may be through screw thread displacement of pre-stressing bias means. Alternatively, activation may be through controlled heat and/or chemical expansion of the fillet member to achieve the desired pre-stressing. Embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings in which; Fig. 1 is a schematic half cross-section of a side and a front view of a rotor core; Fig. 2 is a schematic illustration of fillet members in accordance with the present invention; Fig. 3 is a schematic cross-section of a fillet member associated with a rotor core in accordance with the present invention prior to rotational loading; and, Fig. 4 is a schematic cross-section of the arrangement depicted in Fig. 3 under rotational load. For typical rotor 10 dimensions, filler pieces 1 may have a length to radial depth ratio of about ten and the peak bending stress and radial deflection experienced at the mid-plane of each filler piece 1 could have very high values. These depend upon the rotor 10 design and the geometry of the filler piece 1 cross-section, as well as its density and mechanical properties, but the stress and deflection values would be too high for typical materials, precluding the use of full-length filler pieces 1 in practical switched reluctance electrical machine designs. One approach is to sub divide each filler piece 1 into several fillet members. Each fillet member being of substantially the same length with respective additional support discs or annuli at corresponding intervals in the axial direction across the rotor core 10. This approach is effective in terms of limiting stress and deformation experienced by each individual fillet member, but the need for additional components increases the axial dimensions of the rotor 10 and also the stator housing within which the rotor is secured, so increasing the mass of the machine and reducing its power density. In such circumstances, the use of shorter fillet members can help to avoid excess stress and reduce the deflection occurring at high speeds, but there may still be significant discontinuities in the rotor surface with aerodynamic losses. Finally, the fillet members will experience cyclic stress and strain such that their service lives may be significantly reduced. The present invention seeks to avoid the effects of rotor speed changes by introducing representative pre- stressing levels of bending and shear stress into the filler pieces, and in particular their composite fillet members on assembly, generally similar to the stresses these members experience at the highest speeds for which the rotor 10 is designed. Provided that the materials are chosen to minimise thermal differential expansion, this approach can reduce the relative movement between the filler pieces 1 and the rotor core 10 to negligible proportions throughout the normal speed range of the electrical machine. Minimising such relative movements would help to limit the development of out-of-balance forces and aerodynamic losses, also conserving the fatigue life of the rotor. Taken together, these advantages will allow the length of each fillet member to compose a respective filler piece to be increased, reducing the number of supporting discs or annuli required, with a corresponding effect upon the overall length of an electrical machine. The present invention balances the length of each fillet member with the convenient achievement of pre- stressing in order to resist loading. Generally, the configuration to achieve the best operational performance will be chosen in terms of the actual fillet member length determined by the spacing of the support discs which form the cavity in the rotor core. The fillet member is secured against the value of pre-stressing required to resist expected or specified rotational and other loadings placed upon the rotor core during operation. The requisite pre-stress may be introduced, for example, by the shape of the filler pieces or fillet members such that they are slightly curved along their length in the unstressed condition, but are straightened upon assembly by interaction with the rotor core and the supporting discs or annuli, as illustrated in Fig. 2. Fig. 2 shows a section 5 of the fillet member in an unstressed condition, prior to assembly, with the curvature yr' exaggerated for clarity. In this example, the shape of the cross-section '5a' matches that of the cavity or castellation slot in the rotor core 10 between poles shown in Fig. 1. Fig. 3 shows a partial rotor cross-section along the principal axis after the fillet section 5 has been installed, whilst Fig. 4 shows the same assembly under rotating conditions at the maximum design speed. Referring to Fig. 3, the fillet section 5 is retained at one end by the annular end cap 3, the other end being retained by the intermediate annular component 6. The intermediate component 6 engages the end of the section 5 and is also designed to engage the next adjacent fillet sections in a similar fashion. The dominant mechanical loads are in the radial direction and are indicated by arrows, equal loads ^R1' &λR2' being applied at either end of 5 by the supporting end cap annuli 3 and intermediate component 6 respectively. The loads 1R1' &ΛR2' are reacted by a distributed load applied by a rotor core 20, represented by λLi' 'L4' . The unstressed contour of the fillet 5 may be chosen to achieve a near-uniform load distribution in the axial direction after assembly, in which case 1L1' 'L4' would all be equal. The unstressed contour of fillet 5 may have a variable radius, or a fixed radius may be used, as shown in Fig. 2, if the variation in the load distribution 1Li' , 1L4' is acceptably small. Referring to Fig. 4, when the assembly on the rotor 20 is rotated about the axis AA, the inertial loads experienced by the fillet member section 5 tend to produce additional bending stress and radial deflection in the section. Any such deflection, however, reduces significantly the pre-load applied at the interface with the rotor core, where the elastic characteristics in compression are very stiff. Consequently, at any given speed, a new equilibrium condition is established, between the bending stress in the filler piece on the one hand, and the inertial loads L and the compressive load distribution 1Ls' 'Ls' at the interface with the rotor core on the other. Provided the compressive loads on assembly are at least as great as the inertial loads at the highest rotational speed, then contact will be maintained at the interface with the core, i.e. 1L5' , 'LB' will retain positive values. The deflection in the fillet member 5 occurring between the static and maximum speed conditions depends upon the compressive characteristics at the core interface, but will be small for typically rigid materials, implying that the additional strain will be low compared with that introduced on assembly. The solid cross-section geometry shown at section λ5a' in Fig. 2 is not the optimum, as the minimum deflection under inertial load for a filler section 5 without pre- stress would be obtained by optimising the stiffness/mass ratio. As indicated above, pre-stressing of the insert fillet member 5 in order to create resistance to rotational and other loads is necessary in accordance with the present invention. This pre-stressing can be provided in a number of ways. As indicated above, this may be through shaping such as providing a curved nature to the fillet member 5 such that upon insertion, whether it be slide insertion or snap-fit, the fillet member 5 retains some pre-stressing as described previously. Alternatively, the fillet member may be rendered with a pre-stressed nature prior to operational use of the rotor by insertion of the fillet member into the cavity formed in the surface of the rotor and then providing some in-situ activation of pre stressing. This may be through application of a controlled activation heating and /or chemical process in order to provide sustained expansion of the fillet member to create pre stressing within the cavity formed between the annular members 3, 6 upon the rotor 20 surface. It will also be understood that the fillet member 5 may incorporate a distinct stressing element (not shown) embedded within the fillet member 5, and that this stressing element creates the necessary pre stressing in use of the fillet member 5. Furthermore, only portions of the fillet member 5 may create the necessary pre stressing such that other parts of the fillet member remain relatively stable. By these approaches it is possible to provide the desired pre stressing necessary in accordance with the present invention with the limited range of materials suitable for use within a switched reluctance electrical machine. It will be understood that these materials must be light and non-electrically conducting. It will be understood that generally the pre-stressing of the fillet members 5 in accordance with the present invention will create radial loads against the upstanding intermediate annular components 6 and/or annular end caps 3. The annular components 3, 6 will generally incorporate an overhang for engagement with reciprocal parts of each fillet member. Such engagement may be a simple step engagement as depicted in the figures, or there may be reciprocal tapered ends to the fillet members 5 with a slope upon the annular members or features 3, 6 such that with radial load forces greater engagement between the tapered ends of the fillet member and the overhanging annular features 3, 6 is achieved. As indicated above, generally a cavity is formed between the rotor outer surface and the respective annular features across the width of the rotor and between pole castellations. In such circumstances, the fillet member 5 will have a near flat face to face engagement with the respective upstanding castellation sides of the rotor external surface as depicted in Fig. 1 (b) and an overhanging engagement with the respective annular end cap 3 features and intermediate annular component 6 in order to retain its position and pre-stress of the members 5 and composite filler pieces within each cavity formed upon the surface of the rotor 20. As indicated above, generally the fillet members 5 may be slid into position such that the end cap 3 essentially retains the filler piece assembly formed from a number of fillet members 5 arranged in engagement within a castellation cavity, along with intermediate component features 6. In such circumstances the end caps 3 will generally be secured resiliently to the end surfaces of the rotor in order to ensure retention of the pre-stressing in accordance with the present invention. Generally, it is desirable to provide uniform radial loading and distribution through the fillet members 5 across the rotor 20 in accordance with the present invention. In such circumstances, effective load transfer between the fillet members 5 and end cap annular features 3 and intermediate annular features 6 is achieved. It will be appreciated that weight is a significant factor with respect to installation of fillet members in accordance with the present invention. Essentially these fillet members are there to provide a smooth external surface for the rotor in order to gain windage reduction, thus if these fillet members add significantly to rotor weight, and in particular inertia mass, it would be appreciated that these benefits may be diminished. In such circumstances it is convenient to provide the fillet members in hollow cross-sections, particularly box or I-beam geometries to reduce their weight. Nevertheless, an external surface of the fillet member will be required which achieves a substantially continuous surface across the cavity formed between the castellations in the rotor surface and the annular features as described above. As indicated above, care must be taken with respect to the materials and potential detrimental effects of utilising fillet members in accordance with the present invention. Filler pieces for electrical machines present severe materials challenges. As well as being non-magnetic and electrically non-conducting, the filler pieces must be resistant to the high temperatures arising from aerodynamic losses at the rotor surface and other losses occurring in the rotor itself. In particular, their mechanical properties should be substantially maintained throughout the expected operating temperature range of the electrical machine. Depending upon the rotor speed and the ambient air pressure surface temperatures may exceed 2000C, excluding many plastics materials that otherwise would be suitable, particularly in relation to their passive electrical and magnetic properties. The need for temperature resistance and non-flammability thus tends to restrict the options to a high temperature reinforced plastics material or ceramic materials. Installation in a rotor designed for operation at high speeds implies the use of manufacturing processes giving good dimensional accuracy. The curved forms envisaged for the present invention also emphasise the desirability of near nett shape techniques to minimise production costs. These considerations tend to favour moulding or casting techniques, both of which are applicable to high temperature plastics or ceramics, followed by finish machining and/or grinding operations. The invention requires some means of applying the pre¬ load stressing needed to straighten the filler pieces during assembly. Various methods may be employed, involving either external tooling, features embodied in the rotor design, or some combination of the two. For example, external clamps may be used to encase the rotor assembly, applying generally radial loads to the ends of the filler pieces sufficient to allow engagement with the annular components. Alternatively, the ends of the filler pieces may be tapered to allow progressive engagement with the annular components if the latter had a corresponding frusto-conical mating surface and were installed in a generally axial direction. Usually, the assembly technique should be compatible with a rotor having a laminated core and built up in layers in the direction of the principal axis. Such configurations would generally include some means of securing the assembly, typically by applying an axial clamping load via the shaft or by the use of axial tie-rods passing through the rotor core and acting upon the annular end-caps, 3 & 4 in Fig. 1. In view of the above, it will be appreciated that typically the fillet members will generally be shaped in order to provide a configuration which upon installation in the cavity formed upon the exterior surface of the rotor creates the necessary pre stressing for resistance to rotational and other loads occurring in the rotor during operation as part of a switched reluctance electrical machine. This shaping is typically achieved by a curvature in the fillet member to create an effective arch or bow. However, alternatively the fillet member may adopt a wave shaping or have a varying thickness in order to create the desired pre stressing and loading. It will also be understood that each fillet member may comprise layers of differing material with different reactions to heat and compression in order to create the desired pre stressing for operation in accordance with the present invention. Thus, a more rigid layer of material may be sandwiched between relatively compliant layers of material in order to create a better seal across the cavity and therefore smoother external surface or vice versa with resilient layers on the outer parts of the fillet member with a more compliant layer in between. Alternatively, a fillet member may be formed with a simple two layer combination of a more resilient material on top of a more compliant material again to provide operational performance in terms of achieving a better seal across the cavity and therefore smoother external surface and/or better achievement of the desired pre stressing within the cavity for retention despite rotational and other loadings upon the rotor core in use. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.