Thanks for the comments Guy's
Guiding with internal chip is great, no flexure problems to develop.
KenGee, never had a slab, had compressed sheeting originally, found the cold seeped up through the floor, now have wooden decking with a sheet of 12mm C/D ply over the top, is excellent. Don't worry, have already had a person who shall remain nameless
, wanting my gear, he hasn't offered me enough money yet
.
Cheers
John G