Sorry Peter, I suspect it's a back to Williams job for their auto electrician to set to with his test kit, the advantage with a dealer is they can swap parts with a runner to try and move the fault (really good practice to move the fault rather than just get a car going then your 100% certain you have nailed the problem).
This is one of those that will be very simple open circuit, failed common earth point or something more serious like an ECU I am fearing, hope it's the former.
Please keep us in the loop if it's not too painful, all learning is good.
