Cheers for the input guys, I did actually buy five of the short lengths of 2 x 1 and some metal L brackets but the layout is currently about a week made and is still as flat as a pancake, so i haven't attached them as yet. The cork tiles are unsealed, but i have painted around the edge of the whole layout and the joins between tiles with a strong PVA wood glue and they are held down with double sided carpet tape, so far so good.
I was going to plump for 9mm ply but, erred on the side of caution, it makes it a bit of a heavy b*gger - but I'm really happy with what I've come up with. PS, and it was made in about two hours (including driving time!) Maybe I could start a business....