Yes STW advice is free but you’re just about to get your pants pulled down and professional advice, although an upfront cost will likely mean you saving much more in the long run.
You say he can access the land. This is important information but have you had the title checked? Does the owner have benefit of the access? There may well be a ransom strip preventing him from getting access he may not have the freehold to the access or an options agreement on it.
Is the site constrained in any other way? Does one of the Statutory Undertakers have an easement? Have you checked the Services Map? For all you know there may be a medium pressure gas main under the site making development difficult and material affecting its Potential Use Value.
Have you checked with the Local Planning Authority on their view of its development potential or even if there’s an extant permission in place? All material considerations in negotiations on value of land.
Planning consent will greatly increase its value but if it doesn’t have it and the LPA don’t see it as having development potential then its value is based on its Current Use Value as open space which in the north of England is £10k per acre. If it’s got development potential then who knows what bonkers value it will command down there.
I strongly suggest you either get professional advice or bend over. Your choice!