On Wed, 21 Oct 2020 06:47:10 -0700 (PDT),
wrote:

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.

What does 'concealed' mean in this context? My three downstairs
neighbours' flues pass through my flat to reach the chimneys. They
are behind a wall but I know where they are.