I presume that your symptoms include being able to pull the belt out of the reel easily but it won't automatically rewind. Or is it locked so that you can't pull the belt out?
The wind back spring might have snapped or become detached from whatever it's fixed to inside the seatbelt unit.
Having never been into the seatbelt system, I'd say you're probably looking at a replacement unit, correct seatbelt operation and condition is after all, a testable item on the MoT.
Incidentally, the pre-tensioner is on the buckle end of the belt, fastened to the seat and is triggered by the SRS system to tighten the belt quickly in the case of a head-on. If that has fired, then it is going to be a new unit.