Pictures of BG supply & fit gas wall heater, 1996 at my mother's house. Seen very little use except during last winter (kept on minimum).
When Transco replaced the leaking cast iron main with alkathene, they found 7 leaks in the outside piping, 2 in the inside piping and meter at an "excessive angle" and a suspected 8mm-stuffed-into-15mm elbow (yet to find out if that's the case). That is after emergency visits in 1996 summer re gas leak (internal gas valve, no isolating valve fitted), and again in 1997.
Removing the heater from the wall yielded the following.
Above the heater is an utterly trashed ceiling - it appears moisture in the wall (or combustion vapour leaking) evaporated to condense on the eaves above, dripping onto the sloping ceiling, the internal side of which would be heated by the heater. The location is on a small stair-landing, not where we wanted it, but he had already drilled the hole so a bit late.
Surprised there was such poor attention to detail around the flue fitting.