Postscript - dodgy idle returned! Replaced float valves, as three showed signs of wear, and reset float heights. Tracked it down to a faulty plug, which somehow (!) had become wrongly gapped, and had then destroyed itself. Idle really is well-nigh perfect now, and bike even more driveable - will post on Youtube asap and send the link out.
I guess the lesson is not to take anything for granted, and to be ruthlessly methodical in checking EVERYTHING (like I wasn't).
Regards to all,
Rog