I wouldn't discount a valve issue, I've seen a burnt out inlet valve due to a failed ECU which was causing a poor mixture and subsequently a misfire at idle, so its worth a compression check.
If you do take the distributor cap off, look for any brown "rust" dust this is the first signs of a failing distributor, they are sealed bearing and takes a lot of stripping of a unit that was never actually designed to be stripped, so I would just replace the distributor if in any suspicion of beginning failure, thou this wont normally cause any other symptom until final failure e.g. snapped cambelt or sheared or slipped camshaft drivegear.
Steve M
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Its all gone Norfolk!!!
Now in the Lotus position.