Gadorey/Bess: Thanks for your comments. All the things you mention have been checked out/complied with at some stage.
Dave: You say it’s a long shot but you’ve got me thinking here. The rings on the NOS Hepolite +.030 piston were each slightly pitted in one location and I decided not to use them. At the time I couldn’t find +.030 rings anywhere so on the advice of a couple of trusted suppliers I purchased +.040 rings and gapped them to suit my bore. I gave them the feeler gauge and flashlight check and, if memory serves, I was fairly satisfied that there was good contact with the bore all the way round. It’s just possible I was being a bit optimistic in thinking they would quickly bed into the bore and was reassured by the fact that compression was very good. However, it now seems to be a very possible cause of the oil in the combustion chamber, if not the lack of performance.
My trusty Suffolk supplier now has +.030 rings in stock and I’ve ordered a set – it may make no difference but it’s worth a try. Fingers crossed.