Thanks again dojj - managed to get to the seat belt as you described.
However, the replacement I got also appeared to be jammed, even without the cable attached, so I decided to take it apart - wished I hadn't when the coil sprung out and swiped me across the face - stung a little.
After a couple of hours and many cuts to my fingers I managed to get it coiled back up, and got the seat belt unjammed, however its not returning very well so might have to do more fiddling before the MoT. It pulls out fine, and jams if I pull hard as its supposed to, but I have to keep pulling it hard before the spring seems to activate and pulls it back in a little, I then have to keep doing that until its almost fully rewound. I think I made a mistake taking it apart to be honest.