Try Ambersil
www.amberchemical.com/ they are in Bridgewater, Somerset. They produce a whole range of RTV silicones and are very helpful. We use silicones in school for various DT projects and have used Ambersil for years - brilliant stuff. One thing to be aware of though is that if you are going to make moulds that are under any kind of pressure, you must de-gass the mixture before using it. I know because I have had a few failures before finding out about de-gassing, none since!
Mike