If it is land that doesn't have PP for a dwelling then it's classed as agriculture land.
Google prices in your area for agriculture land. You can usually pop in to local estate agents that have land agents attached.
They will be able to tell you accurately what agriculture land goes for per acre.
They might even be able to write something down for you to give to your developers but might charge for this.
I think it all depends on whether this land could be considered for planning permission. As the access is so poor, I doubt it.
So your price is per acre of agricultural land.
Don't be pricing it as land with PP if they clearly haven't applied, or maybe they have and been turned down. In this case I'd also give your local planning department a call to see the outcome.
We had a garden years ago that was agriculture land, the house was on a border and built with planning permission. We bought the land at the back and took our hedge down, it became our garden. But it remained classed as agricultural land.
At a guess would be the local price per acre divided to reflect the plot size with a few £1,000 as it will add value to your house.