Thanks Gus, useful suggestions.
I haven't had the ecm checked, can't get the plastic cover of the housing because it's got security fasteners.
I have checked EM2 which I understand is the ecm ground and found it to be ok but cleaned and reassembled it anyway and will do the frame ones next.
i have various things to check suggested by guys here and over your side of the pond so I'll have a busy weekend.
I do just have a gut feel that it is something that's just stopped working/broken beacuse it happened suddenly, before then I have had no problems at all. If it were a bad connection or wire I'd expect to have had some intermittent faults before now.