It may have been an expensive classic car ten years ago, but ten years mouldering away in a garage, the engine never turned over, the electrics not checked for mouse nibbling - it's scrap.
I agree with others - put it in writing that they have 28 days to remove the car from your property, or you will scrap it.
"They could move it to their own property but don't want to. It was always meant to be temporary. I have brought it up several times over the years and they just brush me off with excuses every time."
Can I suggest - you weren't firm enough with them all these years? And they knew you wouldn't be, knowing they could brush you off and their car would stay in your garage. So this time - you have to be VERY clear. No polite requests, no 'sorry can you', no ambiguity at all.
For example : 'If the car is still on my property on <date>, NamedScrapyard will be collecting it and I will forward you their invoice, they have quoted me £xxx for their services and I have provisionally booked it's removal.'
And do it.
PS : just googled to see likely charge - first result came up was https://www.sellthecar.co.uk/. You enter the registration number and they will tell you how much they will pay you for the car.
So I now suggest that you send them, in writing:
'If the car is still on my property on <date>, https://www.sellthecar.co.uk/ have offered £xxx and will be collecting it the day after. Whether you pick it up or they do, this car is exiting my garage.'