The problem appeared to be the kickstart lever shaft, 90-88, bearing on the inside of the cover. Luckily, I had another - whicxh gave no trouble - so I fitted that instead. No more problems!
Once started, the engine was so much quieter and seems to run more smoothly, or that may be my imagination odf course.
Anyway, that's another little job done! It just remains for me to give the entire bike a good check over before I take it for the annual MoT next Tuesday!