Just an update - went to look at the clips the local dealer ordered in for me - they're the blue ones I can see part of hiding behind the remains of the trim still attached to the car
Have now had to order a complete new trim (about 70 quid) which does, thankfully, at least come with the half dozen trim clips.
It looks like the remains of the outer trim have been stuck back on rather poorly in the past, so what I've removed is only the very outer layer of the trim - I still have to remove the rest, which I am told slides across like the inner trim cover to get it off the clips.
I'm not going to remove the rest till Saturday as (a) I'll have the new one to compare to by then (b) the weather strip is still sealing the door bottom so it's better off where it is, even though it looks a mess.....