New shape Vectra rear centre seatbelt wont release?

I have a 52 reg vectra and today we needed to use the centre rear seatbelt but it wouldn't slacken when i pulled on it, I folded the rear seat down to check what it could be but instead what was left of the slack belt was coiled up with the rest of it, and as I attempted to return the seat to its upright position the tightend belt made this impossible.