So what do you want out if it exactly? There is no law that says the seller has to take it back and offer a full refund because to be fair some of the blame is yours and the car has been subject to wear and tear. Go back to them and see what they say, they might take it back and return the cash in full, they might give you some cash-back, they might swap it for something else, they might even have a no questions asked returns policy.......you wont know until you ask them. If they tell you to feck-right-off then go and see the CAB or TSO for some proper legal advice, you might have some legal comeback on this but no-one in here seems to be able to confirm this.....plenty of opinions, but little of what appears to be credible information.......barrack room lawyers as mentioned.
Purposely misrepresenting goods for sale is an offence, but the hard part is proving it was done on purpose, if the seller is confronted he will probably say it was a genuine one-off mistake. However the seller might be pulling this trick on a regular basis in an attempt to make their cars more saleable, if they have done it before and other people have fallen for it and complained about the same thing, TS will drop on them from a great height and you will have a case against them
Diplomacy is always the first step, go and speak to the seller, raise the issue and see which way they bounce and remember as it stands they will know more about their legal obligations as sellers than you presently do.