By taking the X3 back for repair, the dealer has admitted it was either faulty or developing the fault prior to your purchase, so he is legally liable either to repair it or refund your money. Write him a letter stating this and if he does not respond immediately, either with a full refund, or with a car equivalent to the X3 to drive while he carries out the repairs, that you intend to take him to Small Claims under Clegg v Olle Andersson (trading as Nordic Marine) House of Lords 2003. Send the letter by Post Office Special delivery, keep a copy, and staple the certificate of posting to the copy so it becomes a Matter of Record for the Court. If you get no response, carry out your threat, bearing in mind that the case could take time and that on getting ruling in your favour you immediately need to purchase a High Court Sheriff's Order to enforce it against the dealer's assets. Then you have to hope he has some.