There is no guarantee that the £12,000 estimate is not a rip-off. Take the matter up with the dealer. Depending on what the problems are, he may be prepared to take the car back, after subtracting an allowance for your three months use of it. You obviously can't expect your full £10,000 back because that would be "unreasonable", as would expecting him to contribute to a £12,000 repair bill on a £10,000 car.