Seat belt trouble

Hi, First visit here - hope someone out there can help;

I have a 2000 (x reg) 1.2 5 door Clio. The offside rear seatbelt has jammed. I can't pull it out or get it to retract properly. Have taken off as much of the plastic cover it feeds into as I can, but can't free it completely, and don't want to risk breaking it (seems to be a sort of plastic peg in a hole and the edge trapped under door trim stopping it). I can just about see the winding mechanism, but can't get to it to try to free the belt.

Any help appreciated please! I'm due to take teenage daughter and friends to first gig in 2 days, so need to get it sorted.