...and some miserable old b*astard probs (Robert Fetcher)...
The mill is active. I think with just that tractor. It's quite possible that a local farmer has an arrangement to store stuff, which looks entirely plausible given your photo. The buildings are owned by an international paper making company, who if they wanted to, could find legal loopholes to avoid tax rather than produce it in an increasingly soggy old mill in Greenfield.
The secca on the site are decent at what they do - which is why so many people get caught - but they're not complete dicks and just tell people to move along when caught. Posts like this could change that. The reason secca are probably so active, is because the site is likely contaminated from all the old chemicals used in the production of cigarette papers. That would be enough to justify the owners of the site paying the sums they do for ongoing security. It's also an attractive punt for metal fairies (who have already made away with some high value swag).
It's a cracking explore, but it's not somewhere you can realistically just walk onto through the front gate and hope to get inside.