Fixed, at least for the moment.    Here are the details for anyone else with
similar problems.

It is possible to get the panel off with the door closed, although a bit
strenuous.   Having said that, I avoided the strenuous bit.

Haynes is accurate in this case.   You pull off the window winder (mine has
manual rear windows) and then remove the front and rear trim securing screws
from the ends of the grab handle.   The rear one is under a cover.   Note
that the rear one drops inside the panel if you are unlucky.   This is not a
problem if you are removing the panel, but if you are replacing it....!

Anyway, they came out.   Haynes then says remove the surround from the door
opening handle and then prise the bottom of the panel away from the door.
This will pull out the retaining pins and you then manoeuvre the panel up
and off.   That would have been the strenuous bit.    In the event, I pulled
on the door opening handle to make it easier to remove the surround
and - - - the bloody door opened - it had cured itself again like last time.
Anyway, I removed the panel, carefully removed the inner insulation sheet as
far as necessary and gave all the door opening and locking components a good
lubricating with silicone lubricant.    At this moment it is all working
fine again but for how long.....?