In reply to Ed Boyter: I don't think the BMC guidebook writers would shoot you down. I think V3 6a etc has merit.
And I like it when they use the nonesensical grade of V3 6b, for a high, sustained problem (Cool Running, to be precise).
All this talk of the hardest move is garbage in bouldering grades. The only system that addresses the hardest move is the Brit Tech. All bouldering grades are about the overall physical difficulty of the problem.
The thing about bouldering grades that makes them not so great is that there's no physco factor, so a highball V3 with a moody landing (which could feel E2 at least) gets the same grade as some sit-start one mover. Only experienced climbers will get up the highball while any indoor boulderer can get up the sit-start, so the grade clearly doesn't actually describe the difficulty.
For highballs, I think the Bit Adj grade (given for without mats) is most useful, e.g. NTBTA is right at E2 6a (or maybe 6b). As long as you know it's a highball, not something to take a rope on, that gives you the right info. Which is why the BMC guides rock so much - they give all sort of combinations to best describe the problem, like E2 6a (V2) and V4 (E2 6a) and so forth. Round of applause (again).