I wont thanks. I will go down tom and ask them for a proper invoice in letterhead etc and the diagnosis first and let you's know how I get on
If you must do this in person, get a lift down with a friend, Don't go down in your Merc.
Seriously, the original problem must be resolved first.
As the law stands, if you want to sell it on, you cannot legally withold any info on a problem that you are aware of.
Although it may be difficult to prove that you knew of a fault, it could concievably cost you more to sell it on than to fix any outstanding issues.