Bought a used car from a dealer and the gearbox failed within weeks

Sold as seen does not remove your rights against a dealer. A motor vehicle trader who sells to a consumer is bound by the Consumer Guarantees Act 1993, and that Act cannot be contracted out of in a consumer sale. The car must be of acceptable quality, taking into account its age, price and mileage. A gearbox that fails within weeks is a strong candidate for a breach, although the dealer may argue that used cars carry a reduced expectation. Get a written diagnosis from an independent mechanic and give the dealer written notice with an opportunity to fix it. Under the Act you may be able to reject the car and seek a refund if the fault is substantial. The Fair Trading Act also prohibits misleading claims about the car's condition. If the dealer refuses, use the Disputes Tribunal for claims within its monetary limit.