In reply to rockjedi12345:
I used 18mm, went for OSB as cheaper than ply (and i like the look it gives), but went for the thicker osb than I would have got in the ply as again it was cheaper.
Just take a bit of care when drilling the holes for the t bolts so they don't splinter too much. I can highly recommend getting yourself a quality drill bit to do this as it saves time and effort. I used this:
http://www.toolstation.com/shop/Power+Tool+Accessories/d80/Wood+Drill+Bits/...
and it was so quick, pulls itself through, you just have to hold on for the ride!