new seat belt means new seat. to quickly get it through mot then remove the seat belt buckle.
to release the seat belt the seat needs to be in the car at a certain angle, have you tried to remove the seat recently? tilt it forward and try to pull the belt, keep trying this with the seat at different heights and it will eventually give, when it does pull it all the way out, lock the seat into its fixed upright positiona nd then let the seat belt retract