Formally reject the car by letter, outlining the reasons, sent to the trader by Post Office Special Delivery, telling him that you will be returning the car to him on a specific date within the 30 day period and that you want a full refund of everything you have paid him or you will sue. Take a copy of the letter and staple to it the Post Office Certificate of Posting so that becomes a matter of record. Assuming the cost of the car was less than £10,000, if the trader does not refund our money you can sue him for it using: www.gov.uk/make-court-claim-for-money