The present invention relates to a waste disposal unit and in particular to a waste disposal unit for waste or used sanitary- products. The present invention also relates to waste receptacles for use in the waste disposal unit. The waste disposal units, which are used in female conveniences for the disposal of waste such as used sanitary products, present a number of problems to contractors and operatives who maintain the conveniences. Typically the waste disposal units consist of a container having a polythene liner bag: when such a waste disposal unit is serviced, the liner is removed and a closure is applied to it, although in order to minimise its volume the bag is first compressed to expel unnecessary air; the container is then washed and sanitised, before a new liner bag is fitted. The need to handle a more or less open bag of such waste products is generally off-putting and the task of washing and sanitising the container is not easy to carry out in a manner which is hygienic to the operative. Moreover, there is a risk of infection from any sharp waste products such as syringes used by drug addicts, which may have been disposed of in the unit: when the cleaning operative compresses the liner bag, there is risk of any such sharp product puncturing the bag and piercing the operative. I have now devised a waste disposal unit, and a waste receptacle for this unit, which overcome the problems outlined above. In particular, the disposal unit is designed such as to have the facility to accept only licensed waste which is clinical waste i.e. sanitary waste and not as is happening with the current units (because of the wide opening shute/flap) to accept waste bottles, sandwich cartons, purses, handbags etc etc. Such an arrangement, unlike the prior unit is particularly useful in helping contractors and customers to comply with the category regulations. In accordance with the present invention, there is provided a waste disposal unit which comprises a housing having an interior compartment to receive a waste receptacle, the housing having a top wall formed with an opening through which a waste product may be passed and into the waste receptacle. Preferably the opening in the top wall of the housing is generally rectangular in shape. Preferably the housing is provided with a lid for closing over the opening in the top wall of the housing, and preferably for closing over substantially all of the top wall of the housing. Preferably the lid is provided, on its underside, with a projection which engages into the opening in the top wall of the housing when the lid is closed, in order to push into the waste receptacle any product which may have lodged in this opening or in an opening in the top of the waste receptacle. Preferably the waste disposal unit comprises an arrangement for preventing its lid being opened unless a waste receptacle is in position within the housing. Preferably this arrangement comprises a locking element which engages at least one catch provided on the lid, unless a waste receptacle is in position, in which case the waste receptacle bears on the locking element is disengage it from the catch or catches of the lid. Preferably the waste disposal unit is arranged for the waste receptacle to be introduced into its housing through an opening in one end thereof. Preferably this opening is provided with a door or other closure, which is preferably lockable to prevent unauthorised access. Preferably the waste disposal unit is arranged for the waste receptacle to be slid into its housing, preferably with opposite margins of a top panel of the waste receptacle supported on runners on the insides of opposite side walls of the housing. Preferably the interior of the housing of the waste disposal unit includes an upper compartment to receive the waste receptacle for use, and a lower compartment for storing one or more waste receptacles awaiting use, the stored waste receptacles preferably being in flattened condition. Also in accordance with the present invention, there is provided a waste receptacle for the above-defined waste disposal unit, the waste receptacle comprising an enclosure having a top panel formed with an opening through which a waste product may¬ be passed into the receptacle. Preferably the opening in the top panel of the waste receptacle is closed by a diaphragm or membrane which is formed with a slit: this closure accordingly allows the passage of a waste product into the receptacle, but then self-closes under its own resilience once the product has passed through. Preferably the enclosure is arranged so that it can be altered between a generally flat configuration against the underside of the top panel of the receptacle, and a relatively expanded configuration ready for use. Preferably the enclosure, or at least its wall between its top and bottom, are of flexible material capable of being folded to provide the flattened condition of the receptacle, or extended to provide the expanded condition of the receptacle. Preferably the top panel of the waste receptacle projects beyond its enclosure along opposite sides thereof, and preferably also at opposite ends thereof. It will be appreciated that when the waste receptacle is in position within the waste disposal unit, the opening in the top panel of the waste receptacle is in register with the opening in the top wall of the housing of the waste disposal unit. Any waste product passed through the opening in the housing top wall will pass into the waste receptacle. If the product lodges in either opening, then the projection on the underside of the lid will push it into the waste receptacle, upon closure of the lid. When the waste disposal unit is serviced, the used waste receptacle is withdrawn and replaced with a new one: it is an easy matter to clean and sanitise the waste disposal unit itself, whilst the waste receptacle forms a fully enclosed container for -A-

its waste products and can be compressed in height, so far as its contents will allow, without risk to the cleaning operative. An embodiment of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is an isometric view of a waste disposal unit in accordance with the present invention; Figure 2 is a side view of the waste disposal unit; Figure 3 is a vertical section through the unit shown with its lid open; Figure 4 is an enlarged, part-sectional view of a top portion of the waste disposal unit, showing an arrangement which enables the lid of the unit to be opened when a waste-receiving cassette is inserted into position; Figure 5 is a similar view of the top portion of the waste disposal unit, showing the arrangement preventing the lid of the unit being opened when the waste-receiving cassette is out of position; Figure 6 is an isometric view of a waste-receiving cassette in expanded condition for use; and Figure 7 is a similar view of the cassette in flattened condition for storage prior to use. Referring to Figures 1 to 3 of the drawings, there is shown a unit for the disposal of waste sanitary products, the unit comprising a housing 10 which is rectangular in plan view, side view and end view. The top wall 12 of the housing 10 is formed with a localised opening 13 which, in the example shown, is rectangular in shape and extends longitudinally of the top wall 12 along its centre. The unit is provided with a lid 14 which is hinged along one of the longitudinal edges of the top wall 12 of the unit and is formed, on its underside, with a longitudinal projection 15 which engages into the opening 13 when the lid 14 is closed, for a purpose which will be described below. The lid 14 is also provided with a pair of hook-shaped catches 16 projecting from its underside adjacent its free longitudinal edge and adjacent its opposite ends, for engagement into apertures 17 in the top wall 12 of the housing 10 when the lid is closed. One end of the housing 10 is open but provided with a door 18 hinged to a vertical edge of this end of the housing: the door 18 is provided with a lock (not shown) for retaining it in its closed position. The interior of the housing 10 is divided into upper and lower compartments by a horizontal partition 19: in use, the upper compartment receives a waste-receiving cassette 30, which is slid into the housing 10 in the direction of arrow D in Figure 1, with longitudinal margins of its top panel 32 rested on longitudinal flanges or runners 11 provided on the insides of the opposite side walls of the housing 10. A number of cassettes 30, in flattened condition, are stored in the lower compartment of the housing 10, stacked one-upon-another: when a fresh cassette is required for use, it is slid out of the housing in the direction of arrow E in Figure 1, from the stack stored in the lower compartment of the unit . Referring to Figures 6 and 7 of the drawings, each waste-receiving cassette 30 has a localised opening 33 in its rectangular top panel 32: in the example shown the opening 33 is rectangular in shape and extends longitudinally of the top panel 32 along its centre. The opening 33 is closed by a flexible diaphragm 34, which is provided with a slit along its centre: the diaphragm 34 thus allows waste products to be inserted through the opening 33 and into the waste cassette, but then closes up. The top panel 32 of the waste cassette 30 is rigid or semi-rigid and forms a closure for a bag-like receptacle 35, which is generally rectangular in plan view, side view and end view when in its expanded condition, but the walls of which are flexible to allow the cassette to be flattened for storage and expanded for use. Figure 7 shown the cassette 10 in its flattened condition and Figure 6 shows it in its expanded condition. It will be noted that the top panel 32 of the cassette 30 projects beyond the opposite sides and also beyond the opposite ends of the receptacle 35. The waste disposal unit includes an arrangement which prevent the lid 14 being opened unless a waste-receiving cassette 30 is in position within the upper compartment of the housing 10. Thus, referring to Figures 4 and 5 of the drawings, a lock plate 20 extends along the underside of the top wall 12 of the housing 10, below the apertures 17 into which the catches 16 of the lid 14 engage when the lid is closed. The lock plate 20 has a downwardly-turned lug 21 at its inner end, against which a spring 22 acts to urge the lock plate in the direction of arrows A, i.e. towards the openable end of the housing 10. When a cassette 30 is inserted into the unit, with the longitudinal margins of its top panel 32 sliding on the runners 11 of the upper compartment of the housing 10, and then the door 18 of the housing is closed to abut the outer end of the top panel 32 of the cassette, the cassette is displaced inwardly for its inner end to abut the lug 21 of the lock plate 20 and slide the latter inwardly against the bias of the spring 22. This brings apertures 23, formed in the lock plate 20, into register with the apertures 17 in the lid 12 of the housing 10, so that the catches 16 are free to enter and leave the aligned apertures 17 of the lid and apertures 23 of the lock plate 20: this condition is shown in Figure 4. If the cassette 30 is removed whilst the lid 14 is closed (the cassette being slid out of the housing in the direction of arrow B in Figure 5) , the lock plate 20 is displaced outwardly (in the direction of arrows A) under the bias of the spring 22, for portions of the lock plate, at the trailing edges of its apertures 23, to engage the hooked ends of the catches 16 and so prevent the lid being opened (in the direction of arrow C) . Referring again to Figures 1 to 3 of the drawings, when a waste-receiving cassette 30 inserted into the waste disposal unit and the door 18 is closed, the opening 33 in the top panel 32 of the cassette is registered with the opening 13 in the top wall 12 of the housing 10. Waste products P to be disposed of may be inserted into the cassette 30 through the opening 13 in the housing top wall 12 and the opening 33 in the cassette top panel, once the lid 14 has been opened. When the lid 14 is closed again, the projection 15 on its underside engages into the opening 13 in the housing top wall 12, to push the waste product into the cassette, should the product have become lodged only part-way through the opening 33 of the cassette. When the waste disposal unit is serviced, it is an easy matter for the cleaning operative to open the end door 18 of the housing 10 and slide out the used waste receptacle, then clean and sanitise the housing itself and replace the used waste receptacle with a new one. The used waste receptacle forms a fully enclosed container for its waste products and can, if required, be compressed in height without risk to the operative.