Stuck Jammed rear seat belt

My rear middle seat belt was jammed and would only keep tightening and making it hard / impossible to lift the seat back up after lowering it to use the extended boot space.
In hindsight I found this was caused by the plastic button bit( technical term) which I assume is designed to stop this had slide round/slipped and allowed the belt to just keep tightening. You will see this if you ever manage to get it released - or check your existing one before it happens. :)

Try pulling to the extreme left or right and you may be lucky and it will come out

So I was going for my MOT and realised I needed to sort it out. I guess technically if one leaves the seats down they may not check the rear seat belts as it is not presented as passenger nearing - but lets not go there.

So ignore all the videos on you tub about dismantling the mechanism. Slide both front seats back as far as you can - then pull off the rear seat cover - there are two clips holding it down - just pull energetically forward and they pop off. This give you some access underneath. The seat belt end is secured to a single bolt under here. I could not get a socket to it as there is a metal bar in the way - so loosen it with a spanner - takes a bit of fiddle - Pop it off and then gently allow the belt to roll back in - then it will...relax? ...and allow you to pull it back out again - you will then find it catches as you pull out on the plastic popper in the middle of the belt - which was designed to stop it from going in in the first place. Little jiggle to either side and out it comes.
Just reassemble as you took apart = making sure you tighten that bolt up nice and secure. In reality I would not use the seat belt until checked by garage - never have- but its as secure as it was in the first place and now goes thought the MOT. Its a single bolt anchor point and there were no additional fixtures. I will now get it checked at the garage.

Please make sure you get this checked by a professional ASAP as its a safety feature and we dont want anyone hurt.

Do not try taking apart the seat belt mechanism - its difficult and will never be safe again. Its very interesting but not worth a death.