In article , "NotMe"
says...
Recently bought a flat in a 1910 tenement block, and as part of its
renovation I plan to move the central heating boiler some 50cm higher from
its current position so that I can integrate it into one of the new
kitchen's wall cabinets.

My question is:

How does one go about blocking the 4" hole that will be left from the
current flue? I am not particularly worried about the appearance from the
outside as it is not visible at all (no buildings across the road, etc) -
just want to make sure that the remains weather-proof..

PS: the wall is a cavity wall.

Remove the bricks around the current flue. Remove the bricks where
you want the new flue. But the bricks from the top hole in the
bottom hole. Put the bricks from the bottom hole in the top hole.
Just an idea.