Thanks for that, I am sure you will be right about it being an ignition inhibit relay. I had searched the internet for any information on the 33RA and found something that said it was being used for split charging. I therefore thought that was the usual thing for it to be used. My relay (with two others) is on a bracket attached to the engine and the cables immediately enter a wiring loom, so it looks like a standard set-up for this engine (Lister 3Cyl/water cooled). I have had another look at it today and cleaned the connections, but I will get a replacement relay in case it happens again.