Now my middle rear seat belt won't click in, is it just a straightforward swap of the bit attached to the seat to fix this?

There is a Multi at my local scrapyard right now, so fingers crossed

Are you sure you're trying to click it into the correct buckle? The centre seat has two, and each of the two parts that click in will only go into one of them.

After the above check, if you still need one, make sure the donor one is undamaged, and not been in a collision where it may have been stressed.

With a family car, and children, might be worth checking if anything is in it that shouldn't be. Sweets, chewing gum, plasticine, dead hamster, etc.

A bit more time, more info.
Across the rear seat there are 6 buckles. 3 different types according to ePER.
The outer ones each side are 735260818 (2x)
The inner ones for the outside belts, and the anchor for the centre belt, are 735260817 (3x)
The buckle for the centre belt is 735260816 (1x)
So if that one is not working, you need to ensure that you get the same one froom the scrappy. There does not appear to be a change throughout the whole vehicle life, so that's one good thing. New, they are different prices, but all were around £85 each 2 years ago when my ePER was current.

Thanks both. It's definitely the right one I'm trying, got three children so backseat is full up and the other ones in use already...

I don't doubt something has gotten in it, looking at the state of my poor car's floor