It's not three months, it's six months. See: www.honestjohn.co.uk/faq/consumer-rights/ But this does not give you the automatic right to have a fault repaired, then try to bill the dealer for it. The law says you have to give the dealer the opportunity to fix it or take the car back and give you your money back. But the dealer must make this clear in his terms and conditions. If he doesn’t, in cases like yours, many Small Claims Court judges have been finding for plaintiffs who have had faults fixed against the dealer.