Motorcycle sale gone sour

So I recently sold my motorcycle, the guy came with his friend and dad and checked the bike over. They test rode it, started it from cold, I showed them all the work I had done on the bike to get it to the condition I was comfortable selling it. I had owned the bike for a year and whenever I problem came up it was promptly repaired, I included the list in my advert.
The buyer rode the bike home and I messaged him later in the day to make sure he made it back okay as I was concerned given the distance he had to go and had only just passed his Compulsory Basic Training (CBT). He responded saying he made it back and just for good measure I asked if the bike was still all good to which he replied “yeah all seemed good bro”.

Fast forward a couple of days and he messages me saying the bike has cut out on him while riding on the dual carriageway. He’s made it safely to a stop and got it taken to a garage. He asked me if there was anything I didn’t tell which there wasn’t I was completely transparent with him.
The first garage tells him they can’t do nothing for 2 weeks so he takes it to another which he told me he would ride there and risk it. Now it’s today and he’s messaged me saying a gasket has failed on the head as it’s entirely silicone and that there is no way I have been riding this for a year. The bike had 35000miles which was quite high for a bike but the previous owner had assured me he had the engine rebuilt so I included in my advert “top end and bottom end was done before my ownership”.
He is now demanding a refund and threatening to call the police at me. I once again told him all sales are sold as seen and the bike had been running perfectly in the time I had owned it, and that given he has had it for a few days now he could’ve done anything to the motorcycle.
It’s a shame as I even added him on social media in hope to go on a ride once I get my new bike up and running.

What I want to know is does he have a leg to stand
on?