Lurch wrote:

IANARCGI, but I don't think it does need sleeving. How many gas pipes
under floors do you see sleeved for their entire length then vented
into a room?

Very true. I'd think most such stuff was pre-corgi when a gas fire in
upstairs rooms was commonplace. I'd think the current regulations would
be that such an installation would require the floor cavity to be
ventilated.

If there were no joints within the thickness of the floor, there would
be a miniscule risk of a leak. I still think that legally you should
sleeve it, there is a forseeable and avoidable risk of a leak into the
unventilated floor void. Running gas pipes in an unventilated void is
verboten.