the release clip is in the black plastic plug at the bottom, if yuo look there is a small square hole. if you put a paper clip into it, it will release the plug out. the seat has to be tilted at this point
the seatbelt should now release from being jammed. then spool out the belt as far as it will go, slam the seat back, and then plug it back in at the bottom.
sometimes takes 2 or 3 goes.
i've had to fix a few of these.
the last one i looked at which was one of the last mk2's made, for some reason, ford had stopped fitting this release mechanism! clever or what.
so i had to put in a 2nd hand seat with release mechanism from a scrappy.
if you cant find a scrappy seat the right colour, it is very very easy to swap the original cloth or leather cover over (and cushions and armrest if necessary)