Hmm...that's interesting comments there Pleiades and Stoic Bloke. I was reading about high temp oxidation due to acidic oil last week sometime. Guess I'll have to get myself a supply of litmus paper for periodical checks. I wonder if there's higher than normal temp due to friction of the crank spinning inside the inner race of the main bearing?

Whilst my oil changes may not have always been on time my riding style and use means that the engine generally gets regular smoothish operating conditions. My dealer also suggested that it can be equally detrimental to change the oil too soon.

Anyway, new bearing seems to have done the trick! It might have been a tighter fit (less clearance) than the OEM bearing. Thanks for the Loctite link Stoic - I might give that a go.
And then when the bike's rebuilt I guess it's time to stop *****footing around and just ride it like I stole it!