Thanks, 91kx, it WAS the powervalve after all. It seems that when the top end was re-assembled after the stroker kit, the governor assembly for the powervalve was not put together right. A small pin which goes through the governor shaft, and engages the governor clutch mechanism was missing. consequently there has been no movement by the powervalve since well before I bought the bike.
I can't believe the difference that the exhaust valve makes, it's night and day.
KingBrian, I'd check over your powervalve system, that was the whole issue for me.