[Father Ted]

Found the fault today...I think.
Removed sensor and, with the aid of an inspection camera, checked the ring. And...found it is split.
So I've ordered a new one on ebay and doused all associated bolts with plus gas ready for next weekend.

As for ebay parts.....well, I'm not so convinced as others seem to be. I don't choose the cheapest, but, for example and in answer to the post above, I did buy an Ebay wheel bearing last year (Off side front) for £60.
Since fitting it I've been to Aachen and back (from Merseyside), fully laden, with a caravan on. I've been to Eurodisney and back (no caravan this time), numerous Uni visits around the country and general running around. No issues at all.
I know some are made of plasticine, but I think there is a point where the extra outlay doesn't really gain any extra value.