Sold car, now winning bidder took away car and now says its not fit for purpose or roadworthy and wants refund and costs, who's right?

Sold my wifes kangoo item no. 170929436649. the winning bidder then came to collect, I offered a test drive which he took and he inspected the car and engine bay while the engine was running at a normal operational temperature. He said he was happy with the car and gave me the cash, unfortunately he was quite keen to keep the V5, which he did. I've now informed DVLA/Police of his details. He put the car on his trailer and drove back to Peterborough. To cut a long story short he came back to me saying the cars overheating and other things about gases in coolant and wants me to pay for materials he'll fit but if it doesn't cure the problem, I'll have to collect the car within 3 days and refund in full on top of parts. I didn't know of any problem with the car offered no guarantees or returns and thought sold as seen. I can't see how he can claim all this once he's taken the car away from my view, who knows what he might have tinkered with since he took ownership? He's got 100% + in the 20's, I've got 100% positive over 200. I've opened a case through ebay as I couldn't agree to him fitting parts and then expect me to take it back if not working, who knows what damage he may do as I don't know if he's a qualified mechanic. Anyway both threatening court action, giving all correspondence to my solicitor next week. All getting very messy, who would you say is in the right? Thanks in advance.