hi,
recently had same problem. seatbelt needs replacing. replacement cost is about £50 from dealer.
If time is still an issue for the MOT, then take out rear seat. It has to come out anyway to replace the seatbelt.
Replacing seatbelt - take out large rear seat, its held on by four bolts - two in the rear passenger comp and two you will see when seats folded.(you dont have to remove carpet). Remove seat from car, its easier with two people.
Remove covers over upper and lower seatbelt fixings . Undo large nut at top fixing(you can now open the seat) remove covers at each end of the seat(hinge points). Next bit more fiddly - make sure you have the correct tool for removing the seat hinge bolts (star shaped tool,most screwdriver sets arent large enough, go to halfords and buy set -about £25 for eight sizes because you need the next size up for the seatbelt).
Once you have tool bits remove hinge screws,two one end, one the other. seat should come apart.
Prise off trim from underneath, the seat fabric and padding should come off, revealing lower fixing points.
Undo single star shaped screw and seatbelt should be free.
Remove plastic cover(if you havent done so already) - gently release one side of the small joining plate(fine screwdriver or similiar).