The above problem turned out to be a fault in the wiring loom. The sensor itself was ok.
However, after 4 years trouble free, the issue has returned and now after paying for an investigation my local service centre (Croydon Motor Village) has advised that the only option they can offer (because they don't have the facility to do partial repairs on electrical faults) is to replace the whole loom. Apparently, the engine has been rubbing against the loom and caused wear on the wiring.
They have estimated this would cost around £800 for the parts and a day's labour, so prob about £1500.
They recommended me to contact Gonnella Brothers to see if they could repair without replacing the whole loom, but I've read mostly bad reviews here about them.
Does anyone have any recommendations regarding garages (I'm in South London, SE19) or some other way that I might be able to get this repaired?