Just got back to this job after a few months of distractions. I tiled
the floor in our downstairs bog as part of refitting the whole thing.
The old waste pipe for the sink was a copper jobbie, cemented into the
floor. I cut the tiles/grouted around a new plastic pipe (with a
compression union just above the floor level, to take the new waste)
and sealed this around using silicone.
Coming back to it after a few months, I expected the silicone to have
cured etc but, although it has gone off well and filled the gap on the
tile side, it doesn't grip the plastic at all. I rather expected it
would grip the thing properly, so it would hold while I tightened the
screw thread.

Obviously I now need a Plan B. Is there something I should do to
treat the plastic pipe so the silicone adheres to it, or is there
another preferred strategy for this situation?

Can't see any potential for an angle grinder here, but you never
know.....