Thanks for your replies. My bead is one long piece. I'm not sure what
you mean about using a big chisel - it's wedged under the seal and I
don't think I could use a chisel without cutting off the thin ridged
strip that's trapped under there..

I can also see that once I get it out, it's going to be a huge pain
getting it back in again.

My local PVC-U shop quoted me 40 quid for the panel, or 140 quid with
the panel fitted with a cat-flap and into my door, although I'd have
to buy the cat-flap. This seems pretty steep. It would also be nice to
be able to do it myself, because if I ever come to sell the house I
may want to put the plain panel back in.

Peter