Valve stem seal problems usually exhibit themselves on startup. Oil trapped in the upper chamber can leak down onto the top of the piston, resulting in smoke at startup.

Smoke whilst running usually indicates rings, Nicasil bores are very cruel to rings and you can end up with almost 3mm ring gap instead of 0.3mm.

Do you have oil blown up into the air box? Although not smoking, a while ago I used to get oil in the air box and so had compression and leak tests done. Both showed no problem but I decided to change the rings to be completely sure and bingo…no more oil.
So on this occasion I found that although everything, even the old rings were within tolerance it still wasn’t good enough.

I have had the airbox off and apart from some small oily mess diectly under the crankcase breather return hose, there was no oil in the box.

I took the plugs out but they look fine.

Someone suggested I do a leakdown test but as I don't have a compressor I can't go ahead.

Just re read my post and “oil in the air box” is a bit vague. Mine threw up this much oil every 100 miles of spirited riding but nothing when pottering around. As I say both tests showed ok (done at service centre) but the cause was certainly blow by.
Just thinking it’s a little presumptuous for me to say it didn’t smoke, how would I know? I was going the other way at erm…say…60? So I’d better say I wasn’t aware it was smoking.