In article ,
"Set Square" writes:

Alternatively, might it be possible to feed it in through an air brick?

....or take a brick out temporarily.
When I replaced my gas pipework, I drilled a new hole through
from the outside anyway as the meter is outside and the old
hole was too small and in the wrong place. I fed all the
lengths of pipe in through the hole, ahead of the final
one which was coupled up to the meter.

I did consider soldering them together as I pushed them in,
but decided it might be hard to get the angle just right, and
any deviation might put extra stress in the joints in their
final positions. So soldering was done in the wriggle space
under the floor afterwards.

--
Andrew Gabriel