Thanks for the thoughts and opinions.

Several incidents over the last few days - always damp. Including Saturday morning following a night in the garage accompanied by the tumble drier. Never had problems in the rain, but the damp/condensation seems to cause a problem.

Will remove HT leads and coily bits, apply hair dryer, and then insulate. I favour chain lube over WD40 - I wreckon it lasts longer and sticks better.