In article , "Lobster"
says...
snip
Anyway - having reinspected the 'project' this morning, I find that
there is certainly no easy way to raise or move the steel pipe; it goes
upwards through the ceiling (mating with the loo upstairs) and then
continues straight up through the roof to a vent cowl. It's all very
rigidly fixed and sealed in place and would be a nightmare to shift it.
So I'm thinking, cut through the pipe about 1m above the floor so I
can move out the bottom section. How would I successfully rejoin the
cut ends - another Kwickfit adapter, assuming there's a suitable one
available?

I'd remove the remaining mortar and cut the pipe down flush, clean
everything very throughly, apply a large fillet of silicone sealant
then build up a ring of mortar so it looks like it did in the first
place.