In reply to bedsforsleepypeople:
I used 12mm birch ply for mine. 18mm is really too heavy hence 12mm and birch ply is much superior to normal ply and is often used in furniture making for this reason. It doesn't have any voids within it like normal ply so holds screws much better, it doesn't splinter and have rough edges so its safer (and won't puncture your thermarest!) and is less prone to warping and delaminating.
It does obviously cost more, about £50-£60 for an 8' x 4' sheet but you'll only need 1 or 2 sheets so its still way cheaper than buying an Amdro.
Its not readily available from B&Q etc so you will have to find a decent timber merchant or builders merchant but you can buy it online if you're struggling, people sell it on it ebay.
Ideally get it from a timber merchant who can cut it to your sizes, mine charged £1 a cut which was well worth it for mm precise cuts of large unwieldy sheets of ply, far neater than you could do with a circular saw or even a DIY bench saw.
I used no more nails and screwfix turbo gold screws which bite beautifully compared with standard screws.
Post edited at 17:05