It's a firm favourite of mine on account of the massive size of the place with all the roads and buildings, woodland, open spaces and the ever present threat that someone might be there. If you can find it when it's empty then it will give hours of interest. The glycerine hill at the north east end is interesting, plus the connected buildings that were cellulose steaming plants. There's several good view points on roofs. The concrete roofs obviously, not the sheet ones. Also beware of crappy old asbestos.
Also avoid parking in remote laybys because the locals and the police will take an interest if you establish a pattern.
And yes, when it's empty the farmer sometimes calls in to check on the cows and sheep. Sometimes via the back gate too.