Hello
Thanks for the advice to date. Having looked at Ray's very impressive layout I am very tempted to follow his lead and use 12mm MDF. Apart from a small amount of condensation on the Velux windows in cold weather the room is very dry. I guess MDF does not mind reasonable temperature variation too much, just damp. And 12mm MDF is certainly a good deal cheaper than ply so a bit of wastage due mistakes would not be too serious.
John