Anyway, after saying that he'd lost the old trim (which was beyond re-using), he eventually found it again. He just put some mastic-like filler in the gap, so the car is good.
So, if anyone is curious what it looks like, it looks like this. If anyone wants the old one (so they can take measurements) I'm happy to send it to you, I'll hold onto it for a few weeks in case of interest, then it'll be binned.
The saga is mixed up in this thread.