Buying from a dealer
You now have six months to complain. In the last year or so the legal position has changed significantly as a result of a European Directive; there is now a new Sale and Supply of Goods to Consumers Regulations. This means that car dealers and garages have to prove if they receive a complaint within six months about a car or part they supplied that it was OK when they sold it. Yes, the obligation is on them to prove it was in good condition.
Legally a car sold by a dealer must be of satisfactory quality, or of a standard that a reasonable person would regard as acceptable, bearing in mind the way it was described, how much it cost and any other relevant circumstances. This covers, for example, the appearance and finish of the car, its safety and its durability.
Free from defects, except when they were pointed out to you by the dealer, or that should have been revealed by an inspection. The cars must also be as described, so a one owner car must be just that.
Fit for any normal purpose - so it must be a reasonably reliable car and capable of any tasks you specify, like seating seven passengers, or towing a caravan.
So dealers must describe the car accurately and must not say that the car is a good runner if it isn't. Under the Trade Descriptions Act the car must be accurately described to you, whether it is in conversation, or anewspaper advertisement, otherwise the Office of Fair Trading can take out a criminal prosecution. Once you rely on a dealer's description it becomes a term of the contract, such as, 'It's definitely a 1998 model', whereas 'I think it's a 1998 model' would not be a term. If you allow the dealer to put right the fault, however small, you have lost your right to reject the car.
Alternatively you could allow the repairs but put it in writing that you are accepting them 'without prejudice' to your existing legal rights. There may also be a civil action for breach of contract, if some term of the original agreement has not been met, e.g. a radio fitted, or delivery at a certain time. What you could get is a small amount of compensation.
Alternatively, if the dealer is a member of the Motor Agents Association, they may agree to abide by their conciliation and arbitration procedures. Should you be unhappy about the decision, then a further appeal lies with the Office of Fair Trading. A dealer will aiso have obligations under the Road Traffic Act, which means that the car must be roadworthy, unless the car is sold specifically for parts, or breaking.
Warranties
In addition to your legal rights, car dealers will be able to include a warranty. In some cases it could mean a comprehensive manufacturer backed warranty such as that operated by Ford and their Direct scheme, or Vauxhall's Network Q. All dealers offer some form of mechanical breakdown insurance ranging from three to 12 months although the terms and conditions have to be read very carefully, because there can be lots of exclusions.