On Wednesday, 21 October 2020 14:29:10 UTC+1, Andrew wrote:
Concealed flues have also been declared unsatisfactory and
this is catching a lot of people out in flats.
All boxed in with a 'cat flap' every 1.5 metres
where a flue connection was !.

That's okay in a single house. The problem is where the concealed flue runs through a different flat, and the other owner/occupier is not amenable to having it opened up.

Owain