FIRE BARRIERS Technical Field This invention relates to fire barriers and in particular to fire barrier devices typically for use in a building to provide, in the event of a fire, at least a partial barrier to flames, heat, gases and/or smoke in pipes or ducts which penetrate floors or ceilings of the building. The present invention is particularly but not exclusively applicable to pipes or ducts which terminate in a floor waste. Background Art Walls, floors and ceilings of a building commonly have pipes, ducts or service lines formed of a plastics material which penetrate the wall, floor of ceiling and which comprise service lines for example for water or sewerage. In the event of fire however, the penetrating pipes or ducts allow flames and/or smoke and gases to spread rapidly through the wall, floor or ceiling. To overcome this problem, a number of devices are currently known for use in conjunction with plastic service pipes or ducts for closing of the pipes or ducts in the event of fire. Some of the devices which are used are in the form of a collar comprising a heat reactive expandable material or intumescent material which is located on the inside of a steel or plastic sleeve or housing which is placed externally about the pipe. When exposed to heat, the heat reactive expandable material will rapidly expand inwardly to squeeze and collapse the pipe which has softened due to heat exposure. These devices can either be surface mounted or fully or semi-cast into masonry wall, floor or ceiling or can be mounted on one or both sides of the wall, floor or ceiling depending upon the direction of risk. Whilst the above types of device usually function effectively where the penetrating pipes extend through the slab for connection to further pipes, a particularly problem arises in multilevel buildings where the pipe is a drain pipe terminating in a floor waste. Commonly in those situations, the drain pipe is connected to a trap on the underside of the slab or floor. In the event of fire on the underside of the floor or slab, the trap being exposed to heat from the fire is rapidly melted and/or destroyed. As a result flames and/or smoke and gases will pass rapidly through the drain pipe as the trap is no longer in place. The intumescent material in an external fire barrier device which may in place around the drain pipe will expand to crush the softened drain pipe however usually this will not occur sufficiently rapidly to prevent flames and/or smoke and gases passing through the drain pipe to the area above the slab. As a result there is a high risk of the fire rapidly spreading through the drain pipe from one level of a building to the next level. Summary of the Invention The present invention aims to provide an improved fire barrier device for providing at least a partial barrier to the passage of flames and/or heat and gases along a duct or pipe of a building in the event of fire. The present invention in a particular aspect is applicable to a ducts or pipe which passes through a floor or ceiling and which serve as a drain pipe and which terminate in a waste, such as a floor waste or the like. The present invention however is applicable to other forms of pipes or ducts in a building. Other objects and advantages of the invention will become apparent from the following description. The present invention provides in a first aspect a fire barrier device having a main body adapted for location within a pipe, said main body having a flow passage therein for normally permitting flow of fluid through said main body from one side to the opposite side of said main body, said flow passage being closable or being adapted to inhibit the passage of heat, flames and/or smoke through said main body in the event of fire. The fire barrier device in one aspect includes a passage closure member, the closure member being movable from a normal first position in which the fluid flow passage is at least partially open and a second position in which the closure member at least partially closes the flow passage. Means are suitably provided for maintaining the closure member in the first position, the maintaining means permitting the closure member to move to the second position when subject directly or indirectly to heat and/or flames. The maintaining means may include support means for supporting the closure member, the support means normally maintaining the closure member in the first position but releasing said closure member to move to the second position when subject to fire. The pipe is typically connected to a trap and the support means may be adapted for cooperation with the trap whereby when said trap is damaged by fire, the support means releases the closure member from its first position and permits the closure member to move to its second position. Suitably, the support means comprises an elongated member which extends through the passage and has an upper end on or to which the closure member is supported. The elongated member may include aligning means for maintaining the closure member aligned with the passage. The aligning means may comprise a plurality of fingers extending radially from the elongated member for cooperation with the passage. The elongated member suitably has a lower end which is adapted to cooperate with an inner surface of the trap. When the trap is destroyed due to exposure to heat or flames, the elongated member releases the closure member from its first position and permits the closure member to move to its second position at least partially closing the flow passage. In another preferred form, the support means comprise means which seats on the upper side of the main body. Suitably, the support means has one or more openings therethrough for passage of fluid. The openings may comprise a plurality of slots or apertures. The one or more openings is/are suitably aligned with the flow passage in the main body. Preferably, the support means comprises a thin planar member. Suitably the support means is formed of a low flame melt material such when exposed to flames or heat, the planar member will release the closure member. In another preferred form, the support means comprise means between the main body and closure member which normally maintain the closure member in a position such that fluid flow is permitted through the passage. The support means may comprise means within the passage and extending between the wall of the passage and the closure member. Suitably the support means comprises a plurality of circumferentially spaced elements extending between the closure member and passage wall. Most preferably, the elements extend radially between the closure member and passage wall. Suitably, the elements comprise thin or filament-like elements formed of a low flame melt material. In yet another preferred form, the support means comprises mounting means fixed to the lower side of the main body and a support member extending from the mounting means through the passage, the closure member being on the upper end of the supporting member. Suitably the mounting means and support member are integrally formed and the mounting member is suitably adhered or secured to the lower side of the main body. Preferably the flow passage tapers in cross section from the upper to the lower side of the main body and the closure member is of a complementary tapering configuration whereby the closure member in its second position will mate with the flow passage to close the flow passage. The flow passage is preferably of a frustoconical configuration and the closure member is suitably of a complementary frustoconcial configuration. The flow passage suitably extends in an axial direction through the main body and preferably is arranged centrally of the main body. The main body suitably comprises an annular member. In another form not involving a closure member, the flow passage is configured to inhibit the passage of flames, heat and/or gases therethrough in the event of fire. In this form, the flow passage extends at an oblique angle from one side to the other side of the main body to open through the one and opposite sides of the main body to form an inlet and outlet respectively. The flow passage suitably extends diagonally from the one side to the other side of the main body. The flow passage may further open through opposite sides of the main body. Preferably, the inlet is diametrically spaced from the outlet. In another configuration, the flow passage extends in a serpentine manner through the main body. In this configuration, the flow passage has an inlet on the one side of the main body and an outlet on said opposite side of the main body. The inlet may be offset from the outlet or aligned with the outlet. Inhibiting the passage of flames, heat and/or gases will provide sufficient time for another fire barrier device associated with the pipe such as a fire barrier device external to the pipe to be effective and close the pipe to prevent flames, heat and/or gases passing therethrough. The main body may be formed of a heat reactive expandable material or intumescent material. Alternatively, the main body may be formed of a non-intumescent plastics material of other non-intumescent material. The main body may be a relatively rigid material. Alternatively, the main body may be formed of a flexible or resilient material to facilitate its placement into the pipe. The closure member may also be formed of a heat reactive expandable material or intumescent material. Alternatively, the closure member may be formed of a non- intumescent plastics material or other material. The main body suitably has an external diameter substantially the same as the internal diameter of the pipe such that it may engage directly with the interior wall of the pipe and extend transversely thereof. The main body may be secured such as by adhesives or fasteners to the pipe or held frictionally within the pipe. In another form, the fire barrier device may include a holder for holding the main body, the holder being adapted to be located with the pipe for engagement therewith. Suitably, the holder comprises a hollow cylindrical or annular body having an external diameter substantially the same as the internal diameter of the pipe. The holder may be secured by adhesives or fasteners within the pipe or be held frictionally within the pipe. The holder has an inner diameter and suitably the main body has an outer diameter less than the inner diameter of the holder whereby the main body may be received within the holder. The holder may have an inwardly directed shoulder or wall upon which the main body may seat. The shoulder or wall may comprise an annular or part annular wall at one end of the holder, The present invention provides in a further aspect, the combination of a pipe terminating in a floor waste and a fire barrier device as described above, the main body of the fire barrier device being located within the pipe and to extend transversely thereof. The present invention in a further aspect provides a method of preventing or inhibiting flames and/or smoke and gases passing through a pipe penetrating a floor or ceiling of a building in the event of fire, said method comprising the step of providing a fire barrier device comprising a main body having a flow passage adapted to permit the flow of fluid through said main body from one side to the opposite side of said main body, locating said main body with said pipe in a position extending transversely of said pipe, said flow passage being adapted to inhibit the passage of heat, flames and and/or smoke through said main body. Whilst the fire barrier devices are primarily for use with pipes or ducts which penetrate a floor or ceiling of a building, they may be used with other pipes or ducts. Brief Description of the Drawings In order that the invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a preferred embodiment of the invention and wherein: Fig. 1 illustrates in schematic longitudinal sectional elevational view a fire barrier device according to an embodiment of the invention associated with a drain pipe duct terminating in a floor waste and provided with a trap; Fig. 2 illustrates the fire barrier device of Fig. 1 in an actuated position; Fig. 3 illustrates in schematic longitudinal sectional elevational view a further fire barrier device according to an embodiment of the invention associated with a drain pipe terminating in a floor waste and provided with a trap; Fig. 4 is an enlarged sectional view along line A-A of Fig. 3; Fig. 5 illustrates the fire barrier device of Fig. 4 in an actuated position; Fig. 6 illustrates in schematic longitudinal sectional view a further simplified fire barrier device associated with a drain pipe terminating in a waste trap; Fig. 6A is a sectional view of a further form of simplified fire barrier device similar to that shown in the embodiment of Fig. 6; Fig. 7 is a longitudinal sectional view of a fire barrier device installed in a waste pipe according to a further embodiment of the invention; Fig. 8 is an enlarged sectional view along line B-B of Fig. 7; Figs. 9 and 10 are top and bottom views of an alternative fire barrier device with associated holder according to another embodiment of the invention; Fig. 11 is a sectional view along line C-C of Fig. 9; Fig. 12 is a perspective view of the fire barrier device of the embodiment of Figs. 9 to 11; and Fig. 13 is a sectional view illustrating the application of the fire barrier device and holder of Figs 9 to 12. Details Description of the Preferred Embodiments Referring to the drawings and firstly to Figs. 1, there is illustrated a fire barrier device 10 according to a first embodiment of the present invention, the device 10 being associated with a pipe or duct 11 penetrating a floor slab 12 of a building and terminating in a floor waste 13 normally provided with a grating 14. The pipe or duct 11 is connected in conventional manner on the lower side of the slab 12 to a trap 15, in this case a P-trap, usually containing water as a seal as illustrated. As the trap 15 is exposed on the lower side of the slab 12, it will also be exposed to the heat and flames of a fire on the underside of the slab 12. In such circumstances, the trap 15 will rapidly melt and be destroyed such that the penetrating pipe or duct 11 will be exposed directly to heat, flames and/or smoke which will rapidly pass upwardly through the slab and open floor waste 14 to the upper side of the slab 12 therefore greatly increasing the chance of spread of the fire through the building. To prevent or substantially reduce the risk of this occurring, the fire barrier device 10 is provided within the pipe or duct 11, the device 10 including a generally annular main body 16 having a central opening or passage 17 passing axially therethrough. The opening 17 in the illustrated embodiment is of a frustoconical configuration reducing in cross section from the upper to the lower side of the main body 16 of the device 10. The body 16 has an external diameter substantially the same as the inner diameter of the pipe 11 so as to be frictionally retained in a transverse attitude within the pipe 11. Supported above the opening 17 and in substantially vertical alignment therewith is a closure member 18 of a complementary frustoconcial configuration to the opening 17. The closure member 18 is supported by an elongated rod 19 which extends longitudinally downwardly through the opening 17 and pipe 11 into the trap 15. The rod 19 is secured centrally to the closure member 18 by means of a threaded connection or simply by a frictional connection with the rod 19 extending into a bore in the closure member 18. The lower end 20 of the rod 19 is in abutment with a wall of the trap 15 as at 21 to normally maintain the closure member 18 spaced above the opening 17. To maintain the rod 19 in a stable position, the lower end 20 thereof may be provided with a flexible foot or boot 22 which can engage with the trap wall at 21. The rod 19 may also include a series of radially extending fingers 23 which abut or substantially abut the inner surface of the opening 17 to locate the rod 19 centrally of the opening 17 and thus the closure member 18 in alignment with the opening 17. The fingers 23 are relatively thin so as not to constrict flow through the opening 17. Both the closure member 18 and main body 16 of the device 10 are in this embodiment formed of an intumescent material or compound and preferably are moulded by injection moulding from an intumescent material or compound or of a compound containing an intumescent material. Alternatively, the closure member 18 and main body 16 may be formed of plastics or other materials and preferably a material which will resist exposure to heat or flames. The opening 17 is normally open in the position of Fig. 1 and defines a flow passage for fluid flow through the waste 14 as indicated by the arrows in Fig. 1. Thus the waste 14 and drain pipe 11 will function in a normal manner to drain fluids such as water from above the slab 12. In the event of a fire on the underside of the slab 12 which destroys the trap 15, the support to the lower end of the support rod 19 at 21 will be lost. The closure member 18 will thus drop downwardly under the influence of gravity into the opening 17 to act as a plug to the opening 17 as shown in Fig. 2. The flow passage defined by the opening 17 is thus closed and therefore acts as a physical barrier to heat, flames and smoke, this being enhanced by the complementary frustoconical configuration of the closure member 18 and opening 17. Where the main body 16 and closure member 18 are formed of an intumescent material and are subject to heat, they will expand as indicated by the arrows in Fig. 2 because of their intumescent nature to further assist in closing off the duct or pipe 11 and prevent passage of flames, heat or smoke through the pipe 11. Referring now to Fig. 3 there is illustrated a further embodiment of fire barrier device 24 according to the present invention which is similar to the embodiment of Fig. 1 and in which like components have been given like numerals. In this case, however, the closure member 18 is supported above the body 16 of the device 24 and in alignment with the opening 17 by a thin planar disc-like member 25 which is formed of a material which will rapidly melt and/or be destroyed when subject to heat. The member 25 may be a plastic low flame melt material. As shown more clearly in Fig. 4, the member 25 has an outer annular support portion 26 for resting on the upper surface of the body 16 and an inner disc-like portion 27 on which the closure member 18 is seated. The closure member 18 may be attached to the portion 27 by adhesives or a fastener or fasteners. A plurality of spaced radially extending ribs 28 are provided between the inner and outer portions 27 and 26 of the member 25 and define a series of slots 29 therebetween for fluid flow. The ribs 28 are formed integrally with the portions 26 and 27. In the position of Fig. 3, the closure member 18 is supported by the member 25 above the opening 17 so that a flow can occur from the waste 13 around the closure member 18 and through the slots 29 for passage to the trap 15. Thus the waste 13 and drain pipe 11 will function in a normal manner. When however the trap 15 is melted and/or destroyed when subject to heat or flames, the support member 25 will become exposed to heat and will melt and/or be destroyed. The closure member 18 will then drop under the influence of gravity into the opening 17 as shown in Fig. 5 to close the opening 17 and prevent the passage of flames and smoke therethrough. Where the closure member 18 and/or main body 16 are formed of intumescent material, the member 18 and/or body 16 will intumescently expand to enhance the closing over of the opening 17. The closure member 18 and/or body 16 however may be formed of plastics or materials other than intumescent materials which do not significantly expand when subject to heat. In either case at least the main body 16 may be formed of a resilient or flexible material to facilitate its positioning in the pipe 11 when installing from above to enable it to be reduced in size to clear the waste 13. Where the main body 16 and or closure member 18 are not formed of an intumescent material, a conventional fire barrier device may be provided around the pipe 11 either within the slab 12 or externally of the slab 12. An alternative support 25' for the closure member 18 may also be used in place of the support 25, the support 25' in this case comprising as shown in dotted outline a support post which extends upwardly through the opening 17, the support post being secured to or being formed integrally with a transverse member secured to the underside of the main body 16 by adhesives or fasteners. The support post may be of cruciform shape in cross-section or in any other cross sectional configuration to normally stably support the closure member 18 above the opening 17. Referring now to Fig. 6, there is illustrated a simplified form of fire barrier device 30 according to the invention. The fire barrier device 30 comprises a solid disc-like body 31 having a flow passage 32 formed therein, the flow passage 32 extending in a diametrical direction from an inlet where the passage 32 opens through the top side of the body 31 to an outlet where the passage 32 opens through the lower side of the body 31, the outlet being diametrically spaced from the inlet. The fire barrier device 30 is as above a friction fit within the pipe 11. In normal use, the flow passage 32 permits flow from the waste 13 to the trap 15 through the pipe 11. In the event of fire which destroys the trap 15, any flames, heat or smoke will pass along the passage 32. Because the passage is of extended length as compared to a passage axially through the body 31 (as for example in Figs. 1 to 4), there will be a delay before flames, heat or smoke on the underside of the slab 12 pass to the top side of the body 31 and slab 12. Thus other fire barrier devices associated with the drain pipe 1 1 for example on the underside of the slab 12 or within the slab 12 (as shown in dotted outline) will have sufficient time to operate effectively when subject to heat and close the drain pipe 11 or any connecting pipe line associated therewith. In a preferred form, the main body 31 comprises an intumescent material. Typically the main body is moulded from a material containing an intumesent material. Thus if fire or heat destroys the trap 15, the body 31 of the device 30 will be subject to the heat of the fire and expand. Expansion of the body 31 will cause the flow passage 32 to be constricted and then close over as shown in dotted outline at 33 therefore preventing passage of smoke or flames. In addition, expansion of the body 31 due to its intumescent nature will cause the pipe or duct 11 to be fully closed off. In the embodiment of Fig. 6A in which like components to the embodiment of Fig. 6 have been give like numerals, the passage 32 is of serpentine-like nature extending from an inlet on the upper side of the body 31 to one side of the body 31 and back to an outlet on the lower side of the body 31 which is substantially aligned with the inlet. This passage 32 whilst allowing normal flow of liquid through a pipe will again serve to slow the passage of heat, smoke or flames through the body 31 to allow sufficient time for conventional fire barrier devices to operate to close the pipe 11. The body 31 may be formed of any material. If however the body 31 is formed of intumescent material, expansion of the body 31 when subject to heat or flames will close or substantially close the passage 32 such that additional fire barrier devices may not be required. It will be appreciated that the passage 32 may be in various configurations and may for example extend in a circumferential direction relative to the body 31 such as to be of a spiral or helical form or part form. The device 30 may be used in combination with a conventional fire barrier device which may be within the slab to surround the pipe 11 as (shown in dotted outline in Fig. 6) or externally of the slab 12. A conventional fire barrier device however may not be required where the device 30 is formed of an intumescent material. Referring now to Figs. 7 and 8, there is illustrated a further embodiment of fire barrier device 34 according to the present invention which is similar to the embodiment of Fig. 3 and in which like components have been given like numerals. In this case however, the closure member 18 is normally supported partially within the opening 17 by a series or radially extending elements 35 which extend between the member 18 and wall of the passage 17. Thus an annular passage 36 is formed between the closure member 18 and main body 16 to allow liquid to normally pass from the waste 13 to the pipe-line such as a trap below. The elements 35 are formed of a material which will rapidly melt or disintegrate when subject to heat or flames. The elements 35 typically are formed of a plastics material such as a low melt plastics When the device 34 is subject to heat or flames from below, the elements 35 will melt and/or be destroyed. The closure member 18 will then drop under the influence of gravity further into the opening 17 in a similar manner to that shown in Fig. 5 to mate with and close the opening 17 and prevent the passage of flames and smoke therethrough. The body 16 and closure member 18 as above may be formed of an intumescent or non-intumescent material such as a plastics material which may be a rigid material or resilient or flexible material. Whilst the fire barrier devices described above are particularly suited to use within waste or drain pipes or ducts which terminate in wastes, they may be used in other pipes or ducts in a building. Further whilst the embodiments have shown the fire barrier devices in association with a pipe or duct which penetrates a slab floor or ceiling, they may be used in association with a pipe or duct which penetrates any floor or ceiling. The fire barrier devices described above may be in various configurations other than that described in the embodiments. Thus the flow passages through the main bodies of the devices may be of various configurations and cross sections. The main body may also include more than one flow passage. The cross section of the flow passage or flow passages are such that the flow through the drain pipe will not be constricted and thus the drain pipe will act in a normal fashion to drain water or liquid from above the floor. Whilst the fire barrier devices are preferably a friction fit within the pipe or duct, the fire barrier devices may be supported in an alternative manner within the pipe such as by means of additional support means or holder fixed to or engageable with the pipe. A typical configuration where the fire barrier device 37 is supported in a holder 38 is illustrated in Figs. 9 to 13, the fire barrier device 37 being similar to the device 30 of Fig. 6. The device 37 has a generally cylindrical or disc-like main body and typically formed of intumescent material. The device is most preferably moulded in an injection moulding process from a material containing an intumescent material. The device 37 is formed with a passage 39 which extends from an inlet 40 on the top side of the device 37 to an outlet 41 on the bottom side of the device 37, the inlet 40 and outlet 41 also opening through respective opposite sides of the device 37. As is apparent in Fig. 1 1, the passage 39 is angled to the longitudinal axis of the device 37 such that the inlet 40 is offset diametrically from the outlet 41. The holder 38 comprises a hollow cylindrical or annular body having a cylindrical wall 42 of an outer diameter substantially the same as a pipe or duct in which the device 37 is to be used and an inner diameter slightly greater than the outer diameter of the device 37. The lower edge of the wall 42 is provided with an inwardly directed part- annular rim 43 upon which the device 37 may seat and be supported. The rim 43 is absent in a region 44 with which the outlet 41 is aligned so that the rim 43 does not restrict flow through the outlet 41. The holder 38 is installed within a drain pipe 1 1 as illustrated in Fig. 13 and adhered in position by any suitable known adhesive such as a contact adhesive or alternatively the holder 38 may be simply held frictionally in position. The device 37 may then be located in the holder 38 with the outer diameter of the device 37 being such that it may be passed through the waste 13 with the grating 14 removed. The holder 38 and fire barrier device 37 however may be installed as one. The device 37 will thus permit the drain pipe 11 to operate as normal with liquid passing into the waste 13 flowing in through the inlet 40, through the passage 39 and out through the outlet 41. In the event of fire destroying the trap 15 however, the device 37 will be exposed to heat and due to its intumescent nature it will rapidly expand to constrict the passage 39 as described with reference to Fig. 6. The device 37 however may have its main body also formed of a non-intumescent material and the passage 39 due to its angled nature will inhibit the passage of flames, heat and /or smoke through the device 37 in the event of fire. In this case, the device 37 may be used in conjunction with a known fire barrier device 44 in the slab 12 so that the device 37 will sufficiently slow down the passage of flames or heat to enable the barrier device 44 to function and close off the pipe 11. The device 37 where formed of intumescent material however may also be used in conjunction with the known fire barrier device 44. It will be appreciated that the fire barrier device 37 and holder 38 may be of various configurations other that that illustrated and described. Further a holder similar to the holder 38 may be used to support the fire barrier devices of the embodiments of Figs. 1 to 8. The fire barrier devices of the embodiments of Figs. 1 to 8 where of an intumescent material are preferably moulded in an injection moulding process using a material containing an intumescent material. The devices however may be formed or moulded in an alternative manner of intumescent material. The terms "comprising" or "comprises" or derivatives thereof as used throughout the specification and claims are taken to specify the presence of the stated features, integers and components referred to but not preclude the presence or addition of one or more other feature/s, integer/s, component/s or group thereof. The above has been given by way of illustrative embodiment of the invention however all variations and modifications thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the appended claims.