I really wish I'd looked this up before trying to fix mine myself.
Mine had been sticking for ages, but usually thumping the back of the seat a few times would release it. However, this stopped working, so I folded the seat forward, tried all ways (except the right way) to release it, but it just got tighter and tighter to the point I could no longer fold the seat back. So I had to cut it out.
My MoT is due next month so I just ordered a belt from a breakers yard. Was a bit surprised to receive a large heavy parcel this morning - I've heard of over packaging but this was ridiculous. However, it turns out they have sent me the back of the seat with the belt still fitted
Unfortunately its a different pattern to my seats otherwise I'd just fit it straight in (it also has an arm rest that mine doesn't). Still tempted to do that though - removing them is going to be a nightmare (particularly as I can't figure out how to get into the seat without cutting the material off - anyone know how to do this?).