Ok, ok... Inundated by the responses to my addition to the question!
I've found someone (in Ipswich) who does seat belts and child anchor points, but he wants to see it before he'll tell me whether it can be done or not, let alone a cost. But I was poking around the back of the car today, and I noticed something a little disturbing... It has no parcel shelf.

Well, it has a parcel shelf, but it's 3mm masonite.

Looks good from above (the masonite's been trimmed with carpet), but no strength whatsoever.

There's maybe 2" of the parcel shelf steel left in place under the rear window, and 1" in place along the back of the seat upright.

I'm amazed it was given RWC like that (it was like that when I bought it, I haven't cut steel on it).
So... Does anyone have recommendations as to what I should do with this situation..? Obviously there's a significant amount less strength across the top of the rear seat.

And I have no idea how this guy could mount a belt without it going right the way to the bottom of the boot floor, which would probably then be located off the floor and rear panel join, which would destroy boot space if I have anything connected to the anchor point.

Any (useful!) advice appreciated!
I'll note, this isn't a daily driver, but it's handy to be able to fit everyone in the car when we use it to drop the daily driver to get a service... Since #2 arrived last September, we've not been able to view the Mini as alternative family transport, the way we could back when we only had #1 (who's big enough that a booster is fine).