Do I have to take my sold car back?

My partner sold my car whilst I was away on a course, the buyer was very keen, left a deposit. He took the car for a test drive and liked it and came back with the rest of the cash (Car sold for £275) There was no Tax or MOT as I have another car (Tax and MOT expired at the end of June)
He has had ample opportunity to get the car inspected (no pressure from us to buy!) and he took the car away last Friday, he took it to the garage on Monday and they have basically condemned the car. Said it needs tons of welding and looks like it has been sat in water for months as it is so rusty underneath! I have been driving this car daily until I got my new car last week!

He has now been round saying he wants his money back, despite signing an AA Bill of sale that I downloaded from the AA site which states "Sold as seen, no inspection"

Obviously, I don't want this car back but I do feel really bad as he was buying this for his sons first car so wondered where I stand legally?
I had paid for a lot of welding to be done (Privately) two years ago and expected that to be that!

The V5 is all completed in his name too now! Any advice would be greatly appreciated!
Thanks