I started the strip down to find the cause. Primary chain and clutch - all okay. Generator - all okay. Oil pump - uh oh. What's all this silver stuff in the oil? Cam cover off. Here's the problem.

At the end of the crankshaft is a nut which retains the cams and a gear which drives the oil pump. The nut is secured by a peened tab washer. We'd missed this and had not peened the washer. The nut came undone, travelled outwards on the crankshaft and started drilling into the alloy cover (see pic left). The oil pump obligingly picked up all the soft alloy swarf and pumped it around the engine.

I repaired the damage to the cover, stripped the motor down to the last nut and bolt and washed it out. Rinsed the oil tank and assembled the whole bike with a new set of gaskets.