Thanks John CL, That gives me a couple of things to try.
To recap, I have recently re-honed the cylinders, reconditioned the head - new guides and valves done by a very reputable workshop and head crack-tested. However, that doesn't necessarily solve the situation if there is a score in the head guide bore so that is still a possibility. The guy used a sealant - not sure what but used to help bed in guides, so should have sealed any score one would think.
Edboy, I also replaced the rings with Hepolite from SRM. Unlike my unmarked originals, the top of the rings were marked with a "T", so I can eliminate that possibility. As the bore had only 40 miles from the new rebore, there was no difference in ring gap at any position in the bores.
First step now will be to remove the exhaust pipes and have a decent look for oil sources. I'll plod along and get back to you before too long.
Cheers Col