We were doing nothing illegal. We were there with the land-owner himself so no problem there.
In my opinion, the only thing they could do us for would be excessive noise.
But there's only ever one car going, and it's a long strip. Can't see then getting anything from that.
If it was over the usage, it would've been some council van.
My theory is that it was just two bored plods thinking they'd hit the jackpot. Once they'd confirmed with the land-owner that we were there with permission, they snuck off to hide, in the hope they'd get some of us when we left.