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?

I used plastacine for years a real pain compared to filleting wax. The wax is easy to use and goes hard when left if you used both,iam sure you would opt for the wax. Good luck. 

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

I'd need too much wax. The plug is not large, and one end goes flush to a flat surface but the other end is raised and I need to fill the void underneath it to get a nice shape of mould. I could extend the edges with sign board but it would be very finnicky and take a long time.  Maybe I could use clay but then warm up some wax and put a top layer of wax on the clay?

The trick to using Plastacine is to heat it up in your oven. Almost to hot too handle but not quite. It goes on as smooth as the wax then.
I use lots of plastacine but then I use the wax for the fine detail where the flange meets the mockup surface.The wax is easier to do the fine detail work.
Hope this helps.
Fred

Cheers Fred, yes that helps a lot. Plasticine / wax it is.