Mis sold 2nd hand car! Help!!!?

[Samia]

Hi, Brought a 2nd hand car of a gentlemen which states on auto trader and his website he is a car dealer. Within a couple of days the car losses power and wont start sometimes, phoned the dealer he states his not giving me the money back and quoted 'what are you going to do about it if i dont' mechanic quotes are £800 plus labour to fix the car is only worth £1200 if that. have i any leg to stand on? i need help