Oh, don't scuttle back to lurking!
You make a good point - in real life, you'd pay a surveyor to assess the value of the land and the likelihood of planning permission, and use the report as the basis for negotiation.
What I don't understand is what value the land has if they kept it as agricultural land. If they walked away from the deal, they won't have £900k in their pockets, but presumably the land is yielding several £10k pa for the farm, or is it sitting there unused? (So by selling the land, they are giving up future income)