Owain wrote:

Could you cut and rejoin the pipe in the loft?

I can't easily get at that bit of loft - I can see it, but it's down a
narrow triangle of pitched roof and isn't boarded. I wouldn't want to be
dismantling pipes and so on in there. However, I will be taking out the
whole boxing-in, so I can get at the pipe above the floor-level branch
in question in the bathroom itself.

Finally, when I bashed the hole in the side of the boxed in pipe, I
found it stuffed with fibreglass loft insulation.

It's there to reduce the noise of your turds going down the pipe.

But my turds don't go down that bit of pipe, unless I crap on the roof -
the part the fibreglass is round is above the toilet branch. The
matching boxing-in in the kitchen below (where that might makes sense)
doesn't appear to contain fibreglass, at least not below unit level
where I made another inspection hole. Even if some kind of noise did
travel up the pipe, it's in the bathroom where the toilet itself is
probably going to be noisier. There's no toilet on this stack on the
ground floor.

Still, seems like a reasonable explanation, if misapplied in this
particular case.

Pete