rear seat belt stuck vauxhall corsa ?

My daughter has a vauxhall corsa (X reg) The back seat got stuck and after removing the bolt, we managed to get the seat upright. Now the middle rear seat belt is locked. It has since failed its MOT on just that - the rest of the car is fine! Any ideas on how we can get the rear middle seat belt to release. The other two belts work fine. As the car is not worth a lot of money we do not want to have to spend a fortune just to get this seat belt working.
Thanks for your help