Polymers which may be used to form the polymeric material include polyolefins such as polyethylene and ethylene copolymers for example with propylene, butene, vinyl acetate or ethyl acrylate, polyamides, polyurethanes, polyvinylchloride, polyvinylidene fluoride, elastomeric materials such as those described in U.K. Specification No. 1,010,064 and blends such as those disclosed in U.K. Specificatio