So, after much checking, I drained the tank, filled it with super unleaded (UK high octane) and a good dose of injection cleaner, and it now runs like a dream and sounds magnificent.
Fixed the tickover by adjusting the air bypass screws to one turn out (they were nearly right in), and I've given it several rounds of "Italian Tune Up". Thanks for all your help everyone.
One further thing. Over the years I've had problems with starting, even when warm. The fuel pumps runs for a few seconds, but pulling in the clutch and pressing the start button does nothing. Push the bike back into garage, onto its stand, try one more time and the bugger starts.
Today I've worked out what's up. Sitting on the bike with the front wheel straight ahead and it won't start. On the side stand with the front wheel to the right, and it does. Must be a connection on the clutch switch or starter button cable which only makes when the front wheel is not straight.