Agree, this is when you want a land agent (preferably one that's a divorce lawyer in their spare time!!).Appoint land agent, the contracting company will pay the fee. Insist on at least partial payment up front, I’ve asked for at least 50% of the total projected cost over the project duration in the past. Contractor to cover all costs; legal, agent, reinstatement, re-fencing, your time.
If they’re digging and using your stone/aggregate to bottom the compound it’s £6/t minimum (this will be subject to aggregate tax).
These things are a bit of a hassle but can be a very nice little windfall, just make sure everything’s signed and sealed, construction companies are known to go bust on a fairly regular basis, hence the percentage fee up front.
I did it myself with network rail and undersold myself - land agent every time in future!