From my experience it pays to get permission not only in writing but be quite clear about what you can and, if appropriate, cannot shoot and also if you are the only one being given permission.
Two stories to show you what I'm talking about.
Firstly, I used to have the shoooting rights, including the deer, on a farm which was near a red deer farm.
A few reds escaped and were playing havoc with the farmers turnip fields.
I only found out about it afterwards when he told me about the problem and that he had got the guy who lamped the foxes out to shoot the reds.
No phone call to me
Yes, I know it was his farm but I was paying bl@@@y good money for the stalking and the guy doing the foxes got on for nothing.
Secondly, one of the guys I stalk with had the stalking rights over a small wood which was managed by a forestry company who had negotiated access from the farmer who owned all the surrounding land.
The farmer approached my mate and asked if he wanted to take on the shooting over the farm as well.
After agreeing the rent, the farmer was asked to confirm the agreement in writing but whether for tax reasons or whatever, the farmer refused telling my mate he had the shooting rights, end of story.
My mate then formed a small syndicate and we had three brilliant seasons shooting pheasants etc. with my mate shooting the deer not only in the wood but in the fields.
The farmer knew fine well that my mate was shooting the deer in the fields because he saw him more than once putting deer in to his car parked in the fields away from the wood but never once said anything.
It was only after a couple of years when the keeper on the adjoining estate complained to his boss, ( I'm not going to name either the keeper or his boss who was a proper Lord someone of somewhere) that my mate was shooting "all their deer "that things then blew up.
Leaving aside so called ownership of live wild deer and the fact they were crossing another farmer's fields before going on to our farm to feed on the farmers crops, the boss then kicked up with our farmer.
The bottom line is that the farmer then told my mate that the "shootiing rights " had never included the deer.My mate tod him where to stick his farm.
Sorry if this is so long but hopefully I made my point.
Nick