Was it a condition of sale? Even verbal?
If you bought the car, in good faith, based on what was told to you by the dealer, and it now transpires that they have economical with the truth, then I would argue that on the balance of probabilities (which civil law works), you have a case.
Send them one final letter as you have tried many times to resolve it. Tell them what you've done and how you have tried to work around them, but to no avail. Give them an a final chance to sort it or you give the car back (if that's what you want do) and/or ask for compensation for your time etc if you want to keep the car, especially if it turns out that it doesn't have a history.
Only issue is that said sales man could claim never to have said what he said, but I suspect any paper/email/call trail would tip the balance of probabilities in your favour.
If that doesn't work, county court time.