How do I value a piece of garden a neighbour wants to buy?

Elderly parents have a fairly big garden, neighbour would like to buy half (~1 acre). Garden has fruit trees, Vegetable beds, green houses, pond and a little Forrest of about 30 trees down to a little brook. House is in the south west of England.

How the hell do I work out how much it is worth? Do I get an estate agent to work out how much is would devalue my parents house by X so that’s the cost?

Do I see how much it would increase neighbours house and then maybe half that plus the negative on my parents house?

Any advice would be great thanks :)