There are two types of waste pipe, those for solvent weld and
those for compression. Because the materials are incompatible,
they make them slightly different sizes. So 32mm solvent weld
(ABS) pipe is not the same diameter as 32mm compression (dunno
what it's made of) pipe.


Thanks for the prompt reply.
That would explain things, all the existing is solvent weld.

Looks like I need to go and buy some solvent weld pipe and fittings, it
should be easier than replacing the whole soil stack!