Bill,
Thanks for that.
Just to clarify, I'm not intending to drop a cache on land where permission has been refused, or to try and avoid arranging permission somewhere. I'm just interested in whether these notions about the need for permission are something that is set out in law, or whether it's more for politeness and good relations.
Although it would seem, at first, common sense that a land manager can remove a cache if he finds one that was left without permission, can you confirm whether that is actually the case in a legal sense? Or at least, if it's legal for him to remove it, is he then obliged to return the property to the owner?
In past discussions (I think with the U.S. cachers so it might be different here), I seem to recall that a viable cache cannot be kept by the person removing it; they have to return it to the owner even if it's a lot of trouble. It may not even be legal to remove it if it isn't causing damage and not interfering with the business.
Which makes sense; after all, just because you farm a field doesn't mean that you can confiscate a bag that you see a passing walker cache under a hedge in the field. It would be stealing, just like taking a cache that someone deliberately left would be stealing. You might get away with explaining that you took it away for safekeeping, but it would probably be necessary to leave a note at the site, or demonstrate that you used some realistic means of contacting the owner.
Are there "GAGB guidelines for landowners"? It might be helpful to come up with some (with the aid of your legal department

), giving the official attitude to such matters (what to do if you find a cache on your land / how to decide whether it has permission / options if not /how to get in touch with the owner / how to identify contact details / what you can and can't do with it... etc.).