Dont know much about motor bikes, but the way I see it, the guy took it out for a test drive, 10 mins or so, looked it over and must have been happy with it.
Whose to say he hasnt over revved it all the way home, dropped it, missed gear change etc. Did it have a rev limiter, even if it did he could have bounced it up there.
It must have been to a good standard to get the MoT, when you had it on the rolling road did you have any problems with over heating or the 6th gear slipping???
Think you have been more than fair with the guy. If it did go to court, think you will be able to say that you gave him a refund and he accepted it. He could have rejected it and tried for more cash or return the bike.
Was your partner with you when you were selling the bike, if she heard you say to him, sold as seen, would help your argument, if it goes that far.
Not a great fan of Citizens Advice Bureau, but might be worth having a chat with them.
Shame you got the hassle Jay, as Petet16 says sounds like he is trying it on.
Just had a look at a bike forum and found this:-
http://www.bikechatforums.com/viewtopic.php?p=3231289
Think they point out that as it was a private sale, he had the chance to look it over and accepted it, down to him. Same as your guy.