You're right. If the bore was worn excessively, or out of true, it would smoke most of the time. I guess it takes a few seconds for all the accumulated oil to get hot enough to burn - most of it ends up in the exhaust header the first turn of the engine, I would think, so has to burn off from there.
Thats low miles for a ten year old bike. Can you tell from the old MOT's if its spent a year or 2 doing no miles at all? Valve stem seals are a bit like fork seals, in that they sometimes seem to go a bit crispy and stop working on bikes that dont move for long periods.
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