Good to hear it from the horses mouth, but not really telling us anything we shouldn't already know.
If you are a farmer, then you should know more than your basic rights. You should expect and be prepared for a visit, and record everything that happens yourself, using pen and paper, video (mobile phone), photos, etc.
I've organised a few illegal partie and had to deal with Police on a number of occasions.
Be polite, be quiet, admit nothing, deny nothing. In UK there is no right of silence, but there is a loophole ("I can't remember!").
Nothing scares the police like a video camera, because they then have to folow every letter of the law, which few of them ever do. It is your best insurance against acquittal. I take one to every party I am involved with, and it is the first thing that comes out when anyone sees or hears a siren!
I have never been prosecuted for a drug or party charge, but have been caught red-handed several times. If you keep breaking the law, you probably will get caught, but if you know what you are doing, you can still keep your record clean.
Ch.