I'm afraid trade to trade sales (business to business) aren't covered by the same sale of goods act as retail sales. You could try asking the dealer you bought it off for a goodwill payment or to let you return the car, most likely they will tell you where to go!!.
You only comeback is if the car was mis-described, if they claimed it drives prefectly in the advert you may stand a chance. Proving the dealer knew anything could be tricky as it take 10 mins for the fault to occur. You have no comeback to the customer but it maybe worth a phonecall to him to find out if he told the dealer about it or if he knows what the exact fault is. If you found that the customer took the car into the workshops of the dealer you got it from you may have a case against the dealer.
Try finding out info from the customer first then call the garage.