Private buyer wants money back

I need some advice. I sold 1985 Bedford TK Horsebox, the buyer opted not to view it, despite me saying that he should & that it was available for viewings, nor did he get it mechanically checked by anyone - he's got his own automatic company. I took it out last Friday & did about 10 miles, just to tick it over. It drove beautifully. He'd paid someone to pick it up today at 2pm, knowing that I was leaving at 2:45 & would not be back till late. He paid the full amount in bank transfer today. The driver was running 2 hours late, so the buyer said to leave the keys & paperwork in the truck & he was happy for the driver to collect it in my absence. Then I received a message from him whilst I was away from home, saying it keeps cutting out in second gear, the heating doesn't work & it won't drive faster than 50mph. It's NEVER cut out in second with me, ever. Don't think I've ever tried the heater on it if I'm honest - though he never specifically asked about the heater, so I have not mislead him about this & when I took it out on Friday, it did 65mph, I know because I had to slow down. He's demanding a refund. I signed a receipt saying sold as seen & had left it in the box.

I gave him lots of opportunities to view it, but he opted not to. I honestly & genuinely sold it in good faith & I had never experienced the problems that he's reporting. I'm not s mechanic, I'm a private seller, I told him its mot'd till next July & that it runs great - to me it does, always has done.

I've returned home to find the Horsebox back on my drive & he's demanding a full refund. I've checked the mileage & it's done about 10 miles since I left it.

I'd already posted the DVLA documentation before he contacted me saying he wanted a refund.

Please can you advise me as to where I stand with this.