Lurch wrote:

What have upstairs gas fires got to do with it? Gas pipes are run
under floors in new installations to feed boilers and fires located
downstairs as they aren't put under concrete floors.

With suspended timber floors, the floor void is usually cross
ventilated with air bricks on the ground floor. The main purpose is to
evaporate any water from the ground, but there's no problem installing
gas pipes in such a ventilated floor void.

On upper floors, the floor void is not usually ventilated. You are not
now allowed to install gas pipes in or through an unventilated cavity.
That is what upstairs gas fires have to do with it.