No guys, you are missing one of the key points.
Planning policies in most parts of the country (not sure about National Parks) allow for an additional dwelling to be permitted for a person who derives his income from agriculture or forestry. That, as well as the business about using sustainable materials, seems to be the deal here. Twenty acres is enough to demonstrate a viable smallholding. That was the point of my second reply.
If this groom's full-time job is only looking after the horse, that sounds like accommodation with an agricultural tie to me. And that is what the owners would more easily get, in terms of consent.
And, in turn, that is why are attempting to sale it with hope value, to an unsuspecting punter who doesn't understand these things.
Investment in large bargepole is recommended.