Did the Trader at any point write on your reciept or put a sticker on the dashboard to state that the mileage could not be verified?

If not, then I think you might be able to do something about it, because as a Trader he is deemed to know better, and as a private buyer you have the law on your side.

But im sure it comes down to whether he stated at any time if he could verify the mileage or not.

Go see your local Citizens Advice, and they'll prob advise you to write a polite but stern letter to the Trader first, then suggest small claims court if no joy.