It is indeed difficult to get permission, I know many farmers around here well and they don’t like dogs being run on their land full stop, this could be down to past experience with poachers, them believeing that it will scare game/birds away, or the land already being looked after by a keeper (most are around here) or the simple fact that its a lot easier to say no than yes!
I think your best bet is to get some ferrets and ask to ferret the land first, more often than not they will say yes, give it some time build the trust and then ask to lamp the land with your dog.
Also go along wit