So far I've eliminated the following as the possible cause:
Plugs (replaced)
Air filter (replaced)
Tappets/valve clearances (adjusted)
Carburettor (removed, jet cleaned, one O ring replaced)
The engine actually runs perfectly - even tickover, revs cleanly, no misfire or smoke - but always one plug is black while the other looks to be burning perfectly (brown/grey)
My local Honda agent wants to replace the coils but I can't see how the engine would run so well if one or both coils were faulty.
I'm perplexed!