Following on from my post below, I don't think it's the rotor now that's the problem.

I test started the car on a nice dark night tonight. Where the coil comes out into the engine, there is a thick lead that goes to the dizzy, and a thinner wire held on with a nut. This thin wire sparks when the engine is started, and presumably it shouldn't.

I plan to clean up this wire and its connector tomorrow in the light. Is there anything I should look for particularly, and could you tell this newbie what role this wire plays? Thanks a lot

Tom