A friend of mine was around there recently and scouted all the area.
He said that there usually is no one there, but he's seen a guy in a PVC suit before (but only once). He went to the shed of that mill and then went away. He probably was an asbestos inspector or something like that :3
There's a bridge over the entrance of the mill (main entrance) where there is a small private industrial estate and gym, so be aware that there is CCTV on that side. But there is no hut on the mill itself. Hope you have a good explore if you go :3