I guess I owe the technical guys at the RAC an apology (sorry guys). Late last night my mechanic mate found the relay. Wasn't quite where they said it was as it had come loose
(which is why we couldn't find it). Anyway, despite yesterdays heat, it was sitting in a damp spot. He has re-sited it and (fingers firmly crossed) it seems to be cured.
So, hope this helps someone out in the future if they have a similar problem.