Fair enough..mine will do that if I try to move it too quickly. If I have to grab something from the glovebox in a hurry and make a sudden move in that direction, it locks up. But I suspect that it's supposed to do that..very fast reflex action! Sometimes when I get in and go to put the belt on, if I move too quickly, the same thing happens..I just relax it a bit and try again, a little slower and all is good.
My Pug 406 was exactly the same and it never failed a UK MOT..ever!! In the 10 years I owned it, the seatbelt NEVER rolled back on its own.