Thanks for responding guys.
I've had the bike about 18 months, it has around 6000 miles on the clock and is just about four years old.
I can't say that I've noticed ambient temperature affecting the frequency of the problem and I don't do the clutchless change up thing on this bike.
I rode out for about an hour a couple of days ago and there was only one occasion when there was a suggestion of being reluctant to change up. Air temp around 6 degrees C.
I've being trying to concentrate on each gear change and if I wind the clutch in, in a fairly slow, deliberate manner, it does seem to me that the gears clunk more than I would like. I've experimented with different revs and 'blipping' the clutch lever in and I think if I get it right I get a better gear change - but it's not necessarily repeatable every time. If I change gear in a relatively slow, deliberate manner, I feel the gears 'clunk' as they engage.
As I said initially, clunking into first is nothing new, but I'd expect a smoother change after that - and I just feel it's not as smooth as it should be.