Colin Wilson wrote:
we have some
from St Helens Glass where the seal is simply a strip of neoprene poked
into the gap in the frame, which was overstretched when installed and has
pulled away quite a lot at most corners.

I've had the same thing happen on the internal seals on our windows. You
can just take them out and stretch them back (try an inconspicuous one
first!). A dab of Pritt is sometimes useful for keeping them in place
while they 'settle'.
The outer ones are part of the beading and that has to removed and the
seals slide out (although they havn't receded in the way the inner ones
have).