Andrew wrote:

There are 'old fashioned' balanced flue boilers that are not
fan-forced. I suspect they are the most ineffient types going
and ?most have now been recycled into Nespressos. I regularly
see properties with a modern 4 inch balanced flue adjacent to
a bricked up hole that was about a foot square where the old
balanced flue came out.

Mine's avoiding getting recycled, 78.something% SEDBUK rating last time
I looked, doesn't justify the spend to replace it