Thanks any advice is very much appreciated.
from what I know about the bike it was damaged by the (only) previous owner and while it was damaged and un-rideable, was stolen and loads of parts stolen. I'm pretty sure it has only been ridden by the PO, and as he loved and babied it, I was as sure as I could be it hadn't been abused.
When I removed the lifter shaft it all looked spotless in the engine, and I'm as sure as I can be it had never been opened up before.
There WAS a minuscule amount of wear on the lifter shaft, so I closed it right up and presumed I had found the problem.
I know the PO changed the oil regularly and I'm trying to find out from him what make/grade he used.
Simon