Update on the resolution.
The front end of the o/h's bike had a noticeable *clonk* when braking and particularly when going over speed bumps.
After some intial inspection by Russel Motors (see below) they advised that there was play in the front wheel, and advised new bearings.
The existing bearings had been fitted only 3 years before, and had done less than 1000 miles in that time, so I was dubious that it was strictly a bearing issue.
It was originally Mike (of twins Mike and John) who suggested that I investigate the spacer, as he had once had a fireblade in which had worn the wheel spacer enough, to mean that the bearings were not sitting where they ought to be, and causing excessive wear.
Got the spacer from Kawasaki, and some bearings, and toddled off to the most fantastic bike mechanics in London
http://www.russellmotors.co.uk/.
Stripping the wheel, there was some swarf on the speedo drive-side of the wheel which had evidently come from the speedo drive itself getting slightly mashed (not too bad)

Cleaned that out, and cleaned up the speedo drive.
The existing bearings however, seemed sound (before and after knocking them out).
We were at a bit of a loss at this point as to what was wrong, when we compared the new Kwak spacer side by side with the old one.
Not only were they made of different materials (not a problem) but the new spacer was around 4-5mm longer than the one that was in the front wheel.
There was no signs of wear on the old spacer, so it hadn't shortened or anything.
Put it all back together (after some doubt as to whether the new spacer was too long), and it all bolted snugly back into place.
In short, what appears to have happened is that at some point in the past probably when getting bearings fitted, the wrong spacer has been put back in the front wheel, and the whole front wheel was shifting in the forks causing the knocking sound
I could have spent the next few years 'shelling' out for bearings every few thousand miles, had it not been for the very astute insight of Russel Motors
Top recommendation for them!
