Thanks for replies so far. Now it has been mentioned I fitted a new coil about 2 years ago, before this started to happen. It could be faulty – is there any way of finding out? I have driven the car successfully over the past few days (to get the engine hot) and not had the problem re-occur. I’ll find a spare coil in the meantime and keep it in the boot in case it happens again.
Paul Tullett