Well...I'm sorry to say the issue isn't sorted. I spoke too soon. It's maybe better but not gone completely.
I've now actually made the fuel tap free-flowing (as per this http://www.akhara.com/nc30/fueltapmod/index.html
which seems to be a very similar if not identical fuel tap) and the issue is still there.
Generally the engine pulls really well, the problem is really only noticeable at full throttle. It's a hesitation (quite an abrupt one) around 6k sometimes or around 7.5-8k and also, as I've said before, if changing from large to small then back to large throttle openings. And it's more noticeable in higher gears, definitely. I'd say it actually never does it at part-throttle, only WOT.
Just in case it's a fuel tank venting issue, how does one go about checking this? The longer part of the vent hose is clear, but what's the easiest way to check the tank itself is venting properly? If it is this, it's only a marginal problem as I'm not actually getting full-on fuel starvation.
Could valve clearances being out cause this symptom? I adjusted them all to spec 17,000 miles ago which I know is just over the service interval but really don't expect them to have gone out in that time.
I'm really clutching at straws here, but could the fact that I removed the foam inside the airbox snorkel have caused this? It was all perished so I thought I'd get rid of it. I can hardly believe this would cause this, but I do know how sensitive these bikes are to any airbox changes so I'm not ruling it out!
I'm really struggling to not get a bit despondent about this and don't really know where to try next.