I get your point pinecooler, but your response is a bit ott on the defensive.
Good luck with the cats protection league - you may need to explain the problem throughly as they seem to like to release strays where they've been caught, after being snipped, round here.
My mum got landed with one; cpl lent her a cage trap, caught the stray, but they had no room, so after it had been sorted at the vets they said it would have to be destroyed unless she could keep in a cage at home, just until they got some room and then they'd take him off her... In the weeks that followed, she phoned them, but they never had a space for him so he stayed.
Alternatively, I read a sickening scientific paper from the 1950's or 1960's. Cats were placed individually in rooms. A bowl of food was placed in the room. When the cat first approached the food, a fire extinguisher was sprayed on the cat until it was empty. The food was thereafter replaced daily. All the cats in the experiment starved themselves to death by refusing to approach the food again. So perhaps is you could catch him in your hen shed and bang on the sides and chase him out/water pistols are meant to be good for teaching cats a lesson, he might never go in again?