Well it came off easily, just drilled out the rivet at the top and the bottom (inside by the door rubber).
No rust to be seen underneath, nor paint pealing off which was good. The windscreen rubber itself is getting a bit perished though.
Seems the person who last fitted the trim couldn’t get it on properly and has bent it, and then they’d put loads of mastic behind it to hold it all on (which it hadn’t done very well).
I stripped out the mastic, and put some washing up liquid in the slot in the windscreen rubber to make it a bit more plyable. I then managed (after more than a few attempts) to feed the trim into the slot and hold it in place while I re-riveted the trim back in place – it kept popping out at the top while I was attending to the bottom, and vice-versa.
Anyway, it’s on and better than it was for now. Need to replace windscreen rubber and possibly find better example of trim to fit back in place.