Thanks very much for the offer! That's really kind and I really appreciate it. Unfortunately, I am down in Sussex and I don't think a trip to Wiltshire in 2nd gear is such a good idea! I have the car at friendly local garage and between us, I think, we are getting to the bottom of things.
The car just had a new (2nd hand) gearbox and a new (new) clutch in it, as the old box literally disintegrated taking the clutch with it. Got the new one in and now have this error, so misplaced or dirty sensors would make sense given the transport of the gearbox to me.
I'll try your recommendations over the weekend and let you know how it goes...