Cheers Brit, have spent a bit more time on it today. I ran it until warm just idling and checked a plug, still bright white. That reassured me a bit that maybe it's just the idle mixture that's too lean. So I richened it a little while watching an accurate tacho, idle speed increased (it was a little low) and idle became really smooth. From previous experience it will run better like that but probably right on the limit for the MOT level. Also checked the timing, it was very slightly advanced so put that right which also makes me a bit happier. Then looked at a plug again, and now I see just a hint of creamy/brown colour.
Sorry I don't know the jet sizes, but they should be standard, not aware they've been changed.
One interesting theory I found when googling, is that E5 petrol is now common, although they don't have to tell you it's E5, and E10 is coming. The theory is if a car is set up spot on for regular fuel it will run lean on E5 and E10. Could become an issue for all of us?