Well time for a short update.The car went off to the dealers (Louwman Exclusive) who I think tried quite hard, but could not replicate the fault or identify the cause.
They checked all the earths, examined the data sent the info off to the factory but heard nothing back....The only thing they have identified is that the battery cranking performance is well down on specification - by about 30%. So the battery is now replaced and they are optimistic that the fault will not reappear. It seems, from conversation with the service manager, that they have had showroom cars which have shown the same or similar faults, and used the same solution.
Heading off on tour to Belgium & Germany is a couple of weeks so let's hope so!