You have rights if you buy a vehicle from a dealer new or used, you’re covered by the Sale of Goods Act 1979. This means it must be: Of satisfactory quality, bearing in mind its age, what it cost and how it was described to you, it should also be free from serious defects, other than those you were made aware of.
Listen to the podcast in the Autotrader link, you still have a choice of whether to reject this car and insist on a full refund. If you choose to reject the car as unfit return it immediately, as this normally has to take place within two to three weeks of purchase in the case of a Used car.... follow up your valid reasons confirming the rejection in writing..preferably sent by recorded delivery.
http://www.autotrader.co.uk/CARS/buying/your_legal_rights.jsp
It sounds like a engine management issue, if that's the case the car is a "Lemon" and could always cause you future problems...insist on your full refund if you feel that way. Note: - do not accept their offer to repair the car if you wish to reject, as that will affect your future right to reject it at a future date if the repairs do not cure the problem.