I've removed and unbolted everything else, but can't find a bolt or anything that attaches the seat belt real mechanism to the chassis. I've had a fell, but can't find anything. I might just be being stupid, I haven't had a proper look in daylight (it was dark when I was putting the harnesses!).

Pull all the seatbelt off the reel, it's a single bolt under the spindle normally covered by the belt.

Hi mate.
As Chris says, also make shure you get the angle right or it won't unwind.
Cheers Mark.