I haven't enjoyed what you describe to me as a 'permission visit' yet. All mine are technically civil trespass, meaning the owner can turf you off at best and that's about it.
The thing is the owner's of 19th century mills are rarely about to do that so as long as I can gain access without vandalism it's generally OK.
If you do any more, I would recommend a huge light, like this:
The differences to your photography are massive, and even iphone's get the job done verywell.