How does the centre seat come to bits, as I broke the metal lever that folds the seat (next to the seatbelt catch).

The lever was broken on mine when I bought it. I found it wasn't that difficult to release it by jamming a screwdriver in the catch, but in the end I bought a used seat and swapped it over, just a few bolts on the seat and a few to move the seatbelt from the old seat to the new. The seat I bought was a different colour so I just swapped the cover.

I realise this doesn't quite answer the question you asked. IIRC there were a few circlips holding the latch together, but I decided at the time it would be easier to just swap the seat, as it didn't cost a lot.

The seatbelt guide looks very sad when you take it off though.