WO 2010/121292 PCT/AU2010/000434 1 A DOOR LOCKING ARRANGEMENT INCORPORATING A SECURITY PLATE Technical Field The present invention relates to door locks and particularly relates to a door locking arrangement incorporating a security plate to resist unauthorised access to secured areas. Background It is common practice to secure areas in buildings such as security doors, exit doors, store rooms and the like by use of door locks such as electromagnetic door locks. A typical arrangement is shown in figures 1 and 1A. A door 10 is fitted with a ferrous metal armature plate 16. The armature plate is fitted to the inside of the door and held in place by way of a two-part fastener known as a sex bolt which comprises a threaded countersunk screw 20A which is inserted from the inside (secure side) of the door, and a dome headed stud 20 which is inserted from the outside (insecure side) of the door and is visible from outside the door. Spacer arrangement 18 includes rubber washers to allow the armature to "float" with some small degree of rotational movement. This movement allows the armature to align with an electromagnet 14 which is mounted to the underside or the head of door frame 12. When the electromagnet 14 is electrified, then armature plate 16 is strongly attracted to the electromagnet with a large force to prevent door 10 from being opened. It has been found that locking arrangements like those shown in figure 1 can be prone to attack by way of a core drill or hole saw as will now be described. Referring to figure 2, the locking arrangement of figure 1 is shown, now being attacked with a hole saw 22 mounted in the chuck of an electric power drill (not shown). The hole saw 22 includes a cylindrical outer cutting part, and a central guide drill bit 24. The drill bit 24 is shown being started in the door 10 just below the exposed head of sex bolt 20. The hole saw then continues to cut right through the door 10 and is withdrawn. The result is shown in figure 3. The sex bolt 20 remains holding a small portion of door 10 a to the armature. However, the remainder of door 10 is now free to open and security has been compromised. The arrangement of figure 1 can be attacked in a similar way by use of a grinder or metal saw, to remove the dome head of the stud of the sex bolt. With the head removed, the door can be opened, leaving the armature plate in contact with the lock as shown in figure 3A.
WO 2010/121292 PCT/AU2010/000434 2 Summary of the Invention In a first aspect the present invention provides a security plate for use in a door locking arrangement, the plate including: fixing means for affixing the plate to the inside of a door; mounting means for mounting an armature to the plate; wherein the plate includes hardened inserts which are received in apertures in the plate. The fixing means may include a number of through holes. The mounting means may include at least one threaded hole. The hardened inserts may include rods, ball bearings or discs of hardened material. In a second aspect the present invention provides a door locking arrangement including: a door; a security plate is affixed to the door; an armature is affixed to the security plate; wherein the security plate is hardened, or includes hardened portions. In a third aspect the present invention provides a method of modifying an existing door locking arrangement, the door locking arrangement including an armature affixed to a door, the method including the steps of: removing the armature from the door; affixing a security plate according to the first aspect to the door; mounting the armature on the security plate. The security plate may be affixed to the inside of the door. The security plate may be affixed by way of a number of through holes and threaded fasteners. The armature may be mounted on the security plate by way of a threaded hole in the security plate and a threaded countersunk screw which carries the armature. The method may further include the step of inserting a one way bolt from the outside of the door to be tightened on the threaded countersunk screw. Brief Description of the Drawings An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 illustrates a prior art door locking arrangement; Figure 1A is an enlarged cross sectional detail of the arrangement of figure 1; Figures 2, 3 and 3A show how the locking arrangement of figure 1 can be compromised using a hole saw and/or a grinder or metal saw: Figure 4 shows a locking arrangement according to the present invention; Figure 5 shows the security plate of the locking arrangement of figure 4; Figure 6 shows the locking arrangement of figure 4 under attack by a hole saw; Figure 6A shows the locking arrangement of Figure 4 after attack by a grinder or metal saw WO 2010/121292 PCT/AU2010/000434 3 Figures 7 and 8 show alternative versions of security plates; and Figure 9 shows photographic results of an attack by hole saw on a locking arrangement incorporating the security plate of figure 8. Detailed Description Referring to figure 4, the locking arrangement of figure 1 is shown having been modified to incorporate security plate 30. To do this, armature 16 and sex bolt 20 are removed from the door. Then, plate 30 is affixed to the inside of door 10 by way of inserting screws 33 through countersunk mounting holes 34 in plate (see figure 5) and screwing these into the door. The threaded countersunk screw 20A which carries armature 16 is then attached to and through a threaded hole 36 in plate 30 (see figure 5). In place of the sex bolt 30, a one way bolt 31 with a one way security screw head is inserted from the outside of door 10 and tightened onto the end of the threaded countersunk screw 20A which carries the armature. Due to the shape of the head of bolt 31, once tightened, it cannot be unfastened. It may be necessary to reposition the electromagnet to some degree so that it is properly aligned and spaced in relation to the armature when the door is closed. Referring to figure 5, security plate 30 is shown in more detail. The body of plate 30 can be manufactured from ferrous or non-ferrous material and includes cylindrical apertures 33 which receive hardened inserts 32. The hardened inserts are formed from carbon steel such as steel ASP-23 available, for instance, from Assab Tooling Co Ltd (www.assab-china.com). The carbon steel is purchased in lengths which are cut to form the inserts using a high speed cutting machine. The inserts are then case hardened. Plate 30 further includes fixing means in the form of counter sunk fixing holes 34 for receiving screws to affix the plate to the inside of door 10. Plate further includes a mounting means in the form of threaded mounting hole 36 which is used to mount the armature 16 to plate 30. Referring to figure 6, the modified locking arrangement of figure 4 is shown under attack from a hole saw 22. The hole saw has cut a hole through door 10, but is prevented from further cutting by the hardened inserts 32 in plate 30. Because plate 30 is affixed to the inside of the door by screws 33 in regions outside of the hole cut by the hole saw 22, despite their being a hole cut about bolt 31, the door is not free to open and security is maintained. Referring to figure 6A the modified locking arrangement of figure 4 is shown after attack by either a grinder or metal saw, the head of the one way security screw has WO 2010/121292 PCT/AU2010/000434 4 been removed. However, because plate 30 is affixed to the inside of the door by screws 33 the door is not free to open and security is maintained. Various security plates can be produced in suitable sizes, and with suitable placed mounting holes, to accommodate a range of electronic locks, and can be used to retro-fit to existing locking arrangements. The plate shown in figure 5 is suitable for use with the lock model no FEM5700 available from Fire & Security Hardware Pty Ltd, of Sydney, Australia (www.fshlocking.com.au). Referring to figure 7, another security plate 40 is shown which is suitable for use with lock model no FEM3500, again available from Fire & Security Hardware Pty Ltd. Referring to figure 8, yet another version of plate 50 is shown, this one suitable for use with lock model no MEM2400, again available from Fire & Security Hardware Pty Ltd. The armature of the MEM type locks carry a protruding locking pin and operate on both mechanical and magnetic locking principles. Full details of such types of locks can be found in published patent specification W02008/009057. A laboratory test was performed using the plate of figure 8 and a MEM2400 type lock. The lock was attacked from the outside of the door using a hole saw 22 mounted on an electric power drill. After about 5 minutes of continuous drilling, the lock still held. Referring to figure 9, photographs of the components are shown after the test. Plate 50 is intact with some scuff marks 60 from the hole saw. The hardened inserts 52 are shown removed from the plate, and have only a few scratches. The teeth of hole saw 22 were completely blunted by the hardened inserts 52. In the embodiments described above, the hardened inserts were received in cylindrical apertures or voids in the security plate. In other embodiments, they may be seated in grooves or other recesses in the surface of the plate. In the embodiments described above, the hardened inserts were provided in the form of cylindrical rods. In other embodiments, the inserts may be in the form of ball bearings or discs. In some embodiments the inserts may be free to rotate in the plate. In the case of pins, these may be free to rotate about their longitudinal axis. This makes it even more difficult to damage the inserts, as they spin rather than be worn. In the case of balls, these may be free to rotate about any axis. In the embodiments described above, the hardened inserts were formed from WO 2010/121292 PCT/AU2010/000434 5 carbon steel. In other embodiments the inserts may be formed from a ceramic material and may be provided in the form of pins. In some embodiments, the inserts may be formed from scrap ceramic material. In the manufacture of electronic components waste ceramic material may be generated. This scrap can be used, thus allowing cheap cost of materials and re-use of otherwise waste material. In another embodiment, the security plate is formed by casting an aluminium plate with appropriately positioned holes. Then, a ceramic coating of a material such as A1 2 0 3 is applied to one face of the security plate. The ceramic coating is extremely hard, and resistant to attack by cutting or drilling. In another embodiment, the security plate is formed by casting an aluminium plate with appropriately positioned holes. Then, a layer of steel wool is applied to one face of the security plate. The steel wool fouls a drill or saw attempting to cut the plate by entangling the drill or saw. Any reference to prior art contained herein is not to be taken as an admission that the information is common general knowledge, unless otherwise indicated. Finally, it is to be appreciated that various alterations or additions may be made to the parts previously described without departing from the spirit or ambit of the present invention.