Update: after a lot of fiddling and cursing we finally managed to undo the holding bolt today. I then unbolted the cabin anchor point and belt guide on the seat, and fed the whole lot back through the bulkhead. Even feeding the seat belt clip through required a lot of jiggling, but it went eventually.
I then rang FDTS Ltd, (West Byfleet) as recommended by others on this forum. Sue was AMAZINGLY helpful, and advised that they could fit a replacement retractor to my existing unit, as apparently MMC don't now supply one that fits the space available. She even sent pictures of the replacement, and how to dismantle the old one. Emboldened by her information as to which side housed the main spring (do not open!!) I prised off the opposite side cover and inspected the ratchet mechanism. There was no obvious problem with wear or dirt, but the operation of the ball-bearing lock ratchet was very intermittent.
I therefore posted off the whole kit to FDTS, and they will refurb it for £35 +vat. I completely concur with other TM posters that FDTS are the go-to company for any seat belt issues, good old-fashioned service by knowledgeable and really pleasant people.
Apparently fitting it back in will be an interesting challenge, but that's another story.
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