Thanks for all the replies so far!
The belt actually freed itself for all of a few seconds last night, I hadn't even moved the car or the seat since!
Obviously, it hasn't stayed un-jammed, that would be too easy, and would mean I had accidentally climbed into my Toyota Hilux.
(14 years old, owned for the last 3, 150,000km and still waiting for one single problem!)
Anyway, I digress.... I've had a go at slamming the seat back up, several times with increasing severity (caused by me getting more pi**ed off with it after each slam). Unfortunately no luck there. The latches are engaging correctly on the seat backs, but I assume this shouldn't be an issue as the belt should be free to move when the seat is down anyway - if it isn't, you can't lift it back up!
If I do find a solution, I will post it here. Until then though, I suppose the answer is to carry no more than 4 in the car. Problem solved then!