Thanks to all for their replies so far, especially Golgotha for his thoughtful and complete replye. As to using 2" steel - if you'll cut it and shape if for me - and buy it - and transport, at all at no expense to me, I'm there. 
Other ideas happily welcomed.
Yes, sorry not very helpful was I?
A great material is the stuff sign makers use. There is a guy who makes signs in the unit next to my workshop, and he gives me off cuts sometimes, so asking at your nearest sign makers could pay off. Its basically a sheet of plastic with thin steel laminated over it. Its possible to cut with a hacksaw. But its light and very strong, so will only warp in very large sections. Its about 5mm thick, so as Golotha hates it will have a lip, but that can be disguised.
My own method for warping is to expect it! Wait for the stuff to warp, then remodel the base. Sometimes this can be as simple as adding some filler, especially if its a ruined terrain piece.