As d9 says the vehicle bought from a dealer must be fit for purpose, it must also match the description given (i.e. 1 careful owner when there have been 4) it must be of satisfactory quality (considering its age, history and price paid). The dealer is unable to evade responsibility for mechanical defects even if they were not obvious unless the defects were pointed out and accepted by you. If you had a guarantee, extended warranty, or paid by credit card you may have other additional rights.
First contact the dealer with as much proof as possible, be calm but firm and ask for a refund, repair or replacement and set a time limit, if you are dissatisfied after your visit write and give them 14 days or you will take action in the courts, send you letter by recorded delivery and keep a copy. If the dealer takes no notice write again and head your letter “letter before court action” ask again for a refund, repair or replacement referring to your visit and previous letter.
If you are again ignored commence action in the county court, do not just accept the situation.