One faulty spark plug!!

It wasn't as obvious as I guess it should have been. There wasn't one clear symptom to start with. Ended up swapping plugs about. Un plugging them whilst the engine was running to see if that showed anything obvious. It didn't to start with. Swapped to a new set of HT leads that I have had for a while. That seemed to do the trick.

Drove around a bit, had a blast down the dual carriageway. Bam - back to before. Starting to get frustrating, too many bits and pieces weren't quite adding up, but still convinced it couldn't be anything too serious.

Left it overnight, this morning went to work and did everything non-major that you can do without replacing anything.

Valve Clearances - just because I haven't checked them in over a year..
Re-cut all of the original HT leads to get nice new clean connections.
Cleaned spark plugs - again!
Checked points gap.
Cleaned rotor arm.
Refitted everything.

Started her up - still idling very rough. Something definitely still wrong.

Took the HT leads off one by one. Cylinder 1 CLEARLY made no difference. (when doing the same test previous day had not really been conclusive)

Swapped the spark plugs between 1 and 2, retried. Cylinder 2 made no difference this time.

Swapped that spark plug with a spare. Hurrah - now idling fine.

Took her for a drive, all good. Had a 55-60mph blast down a long straight piece of country road. She handled it fine. Settled back to normal, about town, speeds and got back home with no misbehaviour. Hopefully that is fixed and it all came down to one spark plug!!!