Thanks Jim.
I'll check the sprocket dampers, but chain tension is fine. It's just had a £430 service, and the mechanic should have picked up on all the issues relating to the fact that it has stood idle for so long. It may be that the angle of the gear lever needs some adjustment. Its great of you to write that whole tract of suggestions - thanks!
I'n not really sure how long I will keep the bike for. As it cost £430 to get road worthy, I don't really want to spend too much making it run great, or I will have spent more on it than the bike itself is worth! It is in great nick for its age though. It's only done 36,000 miles. The engine may have been re-built, as I'm told it was bored out to a 600cc.
The gearbox feels ok - I think the gear changer is just out of line.
As you may have guessed, I'm not very mechanically minded, but I have downloaded some service manuals and such in PDF's, so will look at what is best to check (on the cheap!).
Luke