I've been trying to avoid seeking help with this one, in the hope it will correct itself..but of course it won't.
Changing gear has become a problem. Finding 1st and 2nd is fine, as is clicking into neutral when stationary. But 3rd is a bit tricky and needs a bigger shove (more than it used to). 4,5 and 6 get progressively harder requiring a lot of force on the lever to move up.
I've also found that by pulling the clutch lever in as hard as possible, while gripping it very close to it's pivot, makes things a little easier. It's also a tad easier (but not much) to engage the upper gears when the revs are high. Once in each gear everything is fine, there's no slip and changing down is no problem.
I've twiddled a bit with the clutch cable adjuster at the lever but it doesn't seem to have made any difference. Before I get the spanners out I wanted to know what you people think. Cheers.

There is another adjuster at the clutch end of the cable, might be too much slack that the lever adjuster can't sort, check how much free play there is in the lever before the arm on the clutch starts to move.

Beyond that you'll need someone with more mechanical knowledge than me

I had something similar, after a few miles I'd lose 6th, then 5th. It turned out to be the pushrod locknut had come loose and allowed the linkage to adjust itself slightly.

Actually I had a similar problem on my recent trackday so i'll check the clutch cable and see if that makes a difference. It was a new cable so may just need a quick adjustment.

When this bloody rain stops I have another fiddle with the clutch cable adjuster (at both ends if necessary) and check the gear-change pushrod, to see if that makes any difference.
Zippy, I'm interested to hear you had a similar thing happen but how similar? Lower gears ok? Upper gears need a much harder boot?
At least no-one's mentioned gearbox problems (yet). I've heard what a bastard they are to sort out. Any other advice very welcome.

I found it was a problem for a few sessions but was much better when I blipped the throttle. I'll not get to my workshop for a few weeks so will check it all over then.

All sorted. Clutch cable adjustment was all that was needed. Still strange how it was ok one day, then needed a fair bit of adjusting the next. But it's fine now..and the sun's out!