Should be able to help a little: This is all of a website, but i will put it here incase its removed.
Under the law, buying from a trader the car should be: of satisfactury quality, fit for its purpose and as described.
1.There is no law requiring sellers to inform buyers that cars have been involved in accidents or have been 'written off' by insurance companies. However, you could have a claim against the trader for misrepresentation if he does not answer your questions truthfully.
2.If you have been told something factual about the car that made you decide to buy it, but this later turns out to be untrue, the car may have been misrepresented to you. In most cases you can end the contract and have your money back if you act quickly. However, this is a complex area of law. If in doubt get further advice.
Now from my legal point of view (im a tax lawyer ish), it can depend on what you were told, if they said never been in an accident you prob have a case but then again they may not have known. There are a lot of variables that can effect your position such as price and expectations etc etc, so without the full facts its difficuilt to comment.
I always find a letter stating your legal postition and the specific legal details will result in the dealer getting their act together.
Depending on how long you have owned the car may determine their willingness to refund you.
Look at the website its very useful and good to quote and to be prepared. Also remember if the car is over £5000 (check this its been a while since i did) we are talking possible large court costs as small claims is no longer an option.
Edit its late and its been a long day sorry for spelling. Noticed this was bought in september so some aspects are not available now, still get the ball started with a letter.