Genghis,
Thanks for the suggestion. I spoke to a helpful person at P & M this morning who told me that there's a bit of history with HKS 700e engines and valve seats unseating, if I understood him correctly. Because the cylinder heads are oil-cooled, it's possible for oil to enter the combustion chamber through a faulty cylinder head.
Sure enough, after a couple more very rough ground runs today during which the left cylinder remained cool and the right got hot, several drops of oil fell from the left cylinder air cleaner. It was very definitely engine oil diluted a bit with petrol and the only possible way it could have reached the cleaner was through the inlet tract and carburettor.
This would account for the oily plug insulators in the left cylinder, looking quite unburnt. Shining a torch through the plug holes revealed an oily piston crown.
So it looks like a "dockyard job"; one or both cylinder heads off and sent to an engineering workshop to replace seat/s and maybe valve guide/s.
Thanks to all for the quick and considered advice.