Here in the US, Home Depot has PVC tubs for mixing concrete, 24x35 inches, for $12.65. i looked at them as a substitute for a sink and they look pretty indestructible. You ought to be able to fine something like that in the UK.

http://www.homedepot.com/h_d1/N-5yc1...roductDisplay?
catalogId=10053&langId=-1&keyword=mixingtub&storeId=10051#.UGy0dpjA8hU