fig1 a shows a container cover that is generally shown as 1 . the cover comprises an inner portion 2 which stretches over and covers the mouth of a cup a . the inner portion includes a pierceable area which can provide an aperture 9 . the figure shows a container receiving a straw b . the inner portion includes a “ teat ” of material that is not drawn as tightly over the mouth of the container as the inner portion 2 . this allows for a degree of slack in the cover . the inner portion 2 is bounded by an outer portion 3 which can be pulled down the sides of the cup a , so securing the container cover 1 in place . to assist in pulling the outer portion 3 down the sides of the cup there are grip areas 8 . preferably the container cover is a clear flexible member . the outer portion which extends down the sides of the container also provide an added safety feature in that as the outer portions 3 are drawn against the sides of the container due to the inherent flexibility of the material used , if the container is made of breakable material and is dropped the portions 3 prevent the release of shattered material . this is because the sides constrain the material such as glass within the confines of the portion 3 . a container cover is generally shown as 1 in fig1 b . the cover comprises an inner portion 2 which stretches over and covers the mouth of a cup a . the inner portion includes a pierceable area to provide an aperture 9 for receiving a straw b . also included in the inner portion are apertures 5 which allow for venting of the cup . the container cover may includes a teat as shown in fig1 a which provides “ slack ” in the container cover . fig2 shows a container cover having an extended area to form an area of slack 20 with aperture 9 being in proximity thereto . the first portion of the container cover also includes a valve 21 to provide venting of the container cover when in position on a container . fig3 shows a plan view of a circular one way - valve 21 , which has a series of inwardly projecting ribs 21 a . fig4 shows an aperture 9 which may be formed in the container cover according to the present invention . the aperture is provided by a series of lobes , here shown as three lobes 22 , 23 , 24 , which meet at a central area 25 . the lobes may include an adhesive , or be formed of a self - healing material such that when they meet one another , they close the aperture region 25 where the lobes meet . although three lobes are shown as forming the flap valve less or more lobes can be used . fig5 shows the container cover provided with a series of ribs 26 , which assist in gripping of a container on which the container cover is placed and which also provide a pleasing aesthetic appearance for the cover and container . fig6 shows a further embodiment of the invention , where the area of the cover having a main aperture 9 and vent apertures 5 which are bounded by areas of thickened material 4 . as shown in fig1 , the raised dome or teat area which is the area which supports a straw b in a substantially upright orientation , or at a selected angle . the raised dome includes concentric rings 4 of thickened or ribbed material and these form a series of rings around the straw . the rings form raised ribs between which there are areas of thinner material . the thinner material includes apertures 5 , which may be preformed or which may be punched through the material between the rings to provide vent holes . the cover also includes portions 3 which extend down and grip the sides of the container . the side portions include grip regions 6 , which are positioned such that they are toward the upper portion of the container a . the grip regions may include areas of profiling such as raised nodules which provide a secure grip for a person holding the container . towards the edge of the cover is a thickened band 7 , which extends around the base of the cover and consequently around the periphery of the cup a . this band allows for the cover to be pulled down the container and being thickened , minimizes ripping of the cover , which preferably is a natural latex material . the band may also include tabs 8 , which can be pulled to ensure that the cover is fitted to approximately half to three quarters of the way down the cup a . this provides an optimum gripping area to secure the cover on the cup . the cover includes a venting system . as the cover is flexible , if a child was to blow through the straw , the cover would expand and if there was no venting , it could be blown off the cup . further , if the drink in the cup includes or releases steam or a gas , again , if there is no venting system , there is the risk of the cover coming off the cup which could mean that the contents can be spilled . an embodiment of a venting system is shown in fig2 using a valve but the venting system may comprise apertures as shown in fig7 . around the aperture 9 , there are concentric rings 4 which are formed as a rib which encircles the tube or straw . between the ribbed rings , there are thinner areas of material having a series of apertures . the apertures 5 are preferably formed as a ring of apertures in each area between the ribs . the apertures allow for venting of gas from the cup so assisting in the cover being kept in contact with the container . the cover 1 is made from a relatively thin latex material which ideally is between 0 . 018 mm and 0 . 025 mm in thickness , thereby providing a flexible structure . the apertures form areas of weakness which makes the cover prone to tearing and therefore if there is any tearing , by having the ribbing bounding the area ( s ) where the apertures are , this minimizes the risk of the whole cover being torn and hence rendered unusable . consequently , not only does this increase the safety features of the cover , but it also minimizes cost of production as there is a self limiting risk of damage due to the way the ribbing and apertures are positioned relative to one another . the aperture 9 for receiving a straw may be provided as a preformed hole in the cover . alternatively , there may be perforations which would be broken when a straw is pushed through the aperture . again by having ribbing bounding this area , the risk of tearing the whole cover is reduced . once the straw b is pushed through the aperture 9 the edges of the flexible cover cling to the straw , so forming a seal around the straw . by having a dome structure , this provides an area of support for the straw . fig8 shows an alternative rib and aperture arrangement . the cover 1 has inner portion 2 with an aperture for a straw . there is a lattice structure 10 formed of thickened areas of material which also forms the cover . in the diamond shapes formed by the lattice , there are apertures 5 , which provide the means whereby gas from a container on which the cover is placed can be vented . the lattice may be formed as an integral process in the manufacture of the cover or it may be applied and sealed to the cover as a post main manufacturing process . fig9 shows what may be viewed as a combined arrangement of those shown in fig7 and 8 . inner portion 2 of the cover includes an area of weakness 9 . the area 9 , has an area of raised ribs 10 , forming a lattice area around area 9 . the lattice may include apertures 4 but in this case , there are no apertures as the lattice is there to provide an area of reinforcement and strengthening for the cover 1 . there are then two concentric rings 4 forming a boundary around the lattice area . between the rings 4 , there are perforated apertures 5 , to provide for venting of the container . a side view of a similar arrangement to that of fig1 is shown in fig9 . the lattice 10 is provided as a domed region extending from the inner portion 2 of the cover . the dome provides support for a straw that is pushed through the area of weakness 9 . again , concentric rings bound the lattice area and include venting apertures 5 . although the rings 4 that have been shown are substantially circular , it is envisaged that they may be of other shapes , for example , oval , square or irregular shaped . however , what is important is that the ribbed strengthened areas form a boundary around any area where there are venting apertures 4 in the cover . fig1 shows a similar arrangement to that of fig2 . however , in this embodiment , rather than having a skirt region 7 , forming a circumferential skirt around the cover , there is a depending skirt forming a skirt and an integral pull tab 8 which allows the cover to be pulled down the sides of cup a . also shown are grip regions 6 for holding the cup once the cover is in position . there is an area 11 , where advertising material can be printed on the cover , for example a brand name of a drink or the name of the bar where the drinks are being served . it is also envisaged that characters for example film or cartoon characters can be printed on the cover or even customised slogans for the drinker using the cup . although grip regions 6 are shown in fig6 , these grip regions do not have to be present and there can be printing all over the surface of the container and especially down the sides 3 so providing means to advertise for example drink products or outlets supplying the drinks . fig1 shows a dispenser for flexible container covers and in particular , for covers as described for the first and second embodiment of the invention . the dispenser generally shown as 12 comprises a main body 17 having a lid 13 and an aperture 19 at its base . the lid 13 may be hinged by attachment hinges 14 to the body 17 , so allowing the dispenser to be filled with covers 1 . the covers 1 are separated by ridges 16 , which hold each cover in readiness for attaching to a container a . a container a can be inserted into the aperture 19 at the base of the dispenser . handle 18 is operated to bring a cover 1 , which is supported by ridge 15 into proximity with the container a . the container a is then pushed upwards through the body of the dispenser until it reaches a stop 20 which is below the stack of covers . as the cup is pushed upwards from point x to point y the cover 1 is stretched over the mouth of the cup and down its sides and it snaps onto the cup a . although the present invention and its advantages have been described in detail , it should be understood that various changes , substitutions and alterations may be made herein without departing from the spirit and scope of the invention as defined by the appended claims .