200ft x 50ft + flat, if the boundary does not end with some sort of substantial banking this size of area would be a no no IMHO, now air rifles and 12 bore have a very similar effective range, so this leave 3 options, 1, find some one local with a fac air rifle on an open ticket who will take the job on or 2, bring the rats out to a bait point within easy reach of your equipment and 3, find some other land to shoot on, apply for fac for that land, wait 3-5 years for an open fac and then do it yourself
Baiting I think will be your answer!, if thats the case get some tined cat food, squash a stone into the top of the open tin, so the rats can not easily eat the food and place in a safe spot 25-30 yards away, and wait!