NOVA declaration completed. Am considering a pre-MOT next week to see what we need to sort out. Rear seatbelts ARE required in vehicles after 1986. It doesn't have any mounts, so my thoughts are seat removal for the MOT,
IIRC you don't have to actually remove the seat. Simply folding it is sufficient for the belts to not longer be a testable item. Certainly in both 7-seat vehicles I currently own, when the 6th and 7th seats are folded, the belts are not tested.
I'd be very surprised if the car doesn't have welded nuts somewhere for the rear seatbelts. The trim might not allow access to them, but of all the cars I've ever worked on I've yet to see one that didn't have welded nuts for the rear belts. Even if belts weren't fitted (none of my Talbots ever had rear belts fitted, because 1985, much to the fright of any passengers I ever carried.)