It sounds like we have two distinct problems here. If we look at Gavin's original post there is no reference to removing the oil cap etc. so I still think Gavin's problem is wet sumping. Recent runs have started with less smoke because he has used the bike day by day, and it clears on a run. Whilst I wouldn't recommend a one way valve (as supplied by Bri-Tie in the UK) even though the proprietor has sold thousands of these, it has completely cured my wet sumping problem. Furthermore, starting a bike which has wet sumped can cause serious damage, believe me, I've seen it - blown oil seal and burst gaskets due to hydraulic lock in the bottom end, primary chaincase full of oil.
I would be interested to hear an update from Old-Bikes-Are-The-Best who has a different problem where the bike stops smoking when the filler cap is removed. If this genuinely solves the problem then riding round with the filler cap removed would presumably effect a cure, and no!, I'm not suggesting that as a serious permanent recommendation, just a pointer to where the problem lies, i.e tank breathing.