Ed, the more I read, the more respect I have for you guys. There's no
way I would be allowed to spoil the look of the building by sticking a
long flue up another 4 floors, so it seems I could be exempt. The
boiler is on the inside of the exterior wall (and the cylinder and tank
are inside a cupboard about 5 metres away on adjacent internal wall).
I'm still digesting the information you asked me to read. This is a
great way to learn. Thanks.