Re: Engine cuts out at 80mph on M25...AAAHHH!!!
Providing you can demonstrate that you didn't abuse the car, it is the dealer's problem. If you bought the car on hire purchase (not a loan) then it is also the HP company's problem.
Supply of Goods (Implied Terms) Act 1973 Where the creditor bails or hires goods under a hire purchase agreement in the course of a business, there is an implied term that the goods supplied under the agreement are of satisfactory quality.
Your first port of call is the dealer that sold you the car. Make it clear that you regard this as their problem and don't be fobbed off. It may be a good idea to take a witness. Do not under any circumstances say anything that suggests you are prepared to bear any of the cost. If the car is on HP, let them know you are going to contact the HP company.
If this gets you nowhere, you need to speak to Trading Standards at the council that covers the dealer's location. Don't threaten the dealer with this, just do it.
If that get's you nowhere you can take your case to the small claims court. This isn't held in a court room, you sit with a judge and the dealer in an ordinary office and have a chat. This costs you nothing, win or loose, and the Courts Service will help you bring your case.