Paul - I agree about the Hornby 25, which is why I suggested anyone that is inspired to do something similar get a 2nd hand Bachmann 25, with it's much better mechanism, that is if they wanted to spend a bit extra. I can see why Dave has gone for the Hornby one though.
Having been inspired by this myself, I've had a quick peek in the loft to look at suitable building materials and luckily I've found an unused sheet of 4ft by 2ft 3mm thick Harboard. In addition, I've found a fair size chunk of foamboard type stuff (the shop sign type stuff someone else mentioned), some more normal type foam board (ideal for making buildings) and a few off cuts of 2 by 1 timber. That coupled with the insulation board in our garage means I'll just need 4 4ft bits of 2 by 1 to make 2 4ft by 1ft baseboards for a small layout, plus enough material to build a backscene,
thanks
Mike