Guys in the UK, where is the best place to get decent sized chunks of modelling clay? Google isn't helping at all. I could use plasticine I guess but again, I need a big lump of it. I need it to extend from a plug to make the mould a decent shape.  I've never used it for this purpose before and I guess it doesn't need any coating for tooling gel to release from it?

Another vote for wax, if your having to extend a long way, you may be better off adding something more substantial, like if its metal, weld on the extension