Well, I took my GTV to the superb W.A.D in Wolverhampton to try and solve the problem.
Typical isn't it, whenever you take a car to a garage, the problem never shows itself!? One of the mechanics took it for a test drive and it performed normally.
He hooked it up to the diagnostic computer thingy, and it came up with no faults at all.
He then explained that it is quite normal for these V6 engines to experience a surge of extra performance as they pass 3 or 4 thousand RPM, and that it doen't mean that the engine is hesitant prior to this (although a real flat spot is obviously NOT normal).
Anyway, he said it seems and looks ok, but he will try pouring a tin of fuel additive or cleaner (not sure exactly what it was) into the fuel filler, and see if that makes a difference.
Amazingly, it is fine now! I don't what the problem was, but it must be somewhere in the fuel system (blocked injector maybe?).
So, if anyone has experienced this (Aisha, you mentioned something similiar), try getting a can of this stuff from Halfords and emptying it into the fuel tank - it might just make a difference.
Also, I may experiment with Super Unleaded, see if it's any better (as if my fuel bills aren't high enough already - this car has a serious drinking problem, hasn't it???)