Well. it's been a few weeks and the dust is settling. I was able to spend a couple of hours on the bike last night in prep for getting the engine out and down to NLM for a stripdown and report next week.
On draining the oil, there was that heart stopping sound of gravel in a plastic bucket that turned out to be several large flakes of metal dropping into the pan. It look as as thought the main bearing on the crank has disintergrated. Cause unknown, but I suspect corrosion from condensation may have done the deed.
Under the previous owner, the bike had stood in a shed for several years and had only covered less than 100 miles during that period. I'm guessing that a series of very short journeys interspersed with long periods of storage meant that any moisture couldn't get out of the crankcase. The occasional warming and cooling of the oil wasn't enough to get rid of it, and the bearing surfaces may have corroded. Then I come along. I change the oil (which stank of petrol - the previous may not have closed the manual petrol taps, causing fuel to see down into the crankcase through the cylinders) and start riding it around regularly at revs > 6000rpm and hey presto! Crunch!
That's the current theory. Once I get the report from NLM I'll have a much clearer idea. Let's just hope that the premium bonds come up this month

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