The advantage of the fox with no fear is that he doesn't know he's going to be shot so he exposes himself to the hunter! The mystery of our killer is that her den is really quite close to the gamekeeper's house - so the wtf has he been doing for the past three years. He is a man of few words, mostly containing the word "off"
I've seen it recommended to put a wire around the chicken fence at about 9" high and a yard out from the fence - the point from where the fox would launch from to jump the fence. Only snag I can see is a 9" electric trip wire around the pen!