"Nigel Heather" wrote:

Are there any clever options that I am not aware of (bear in mind that it is
10mm pipe)?

You should find the concealed pipework within the cavity wall
will flex an inch or two either way. Pull the pipe out as far as
it will come. Cut leaving enough pipe remaining to apply a
compression end cap, then push the lot back into the cavity.
Apply filler.

Watch that the pipe doesn't slip back into the cavity before
you've got the stop cap on. Use a small pair of pointed mole
grips held against the wall to clamp it in place. If the
plasterboard hole is smaller than the compression fitting tap it
through using a piece of wooden dowel as a drift.

Greg C