Fiat 500 passenger seat belt issue

[FiatT234]

Hello,
I have a Fiat 500.
It’s due for its for Mot and a full service this week.
However, this morning the passenger seat belt stopped working – it’s jammed and won’t pull out – is not freely moving.
It has been fiddly for a few month but we have managed to get it working each time.
However, i am concerned that it may now fail its Mot because of this.
Any ideas on how to get this working smoothly again.
I think it might be the mechanism/retractor....
Unfortunately warranty expired last October...
Many thanks