Around here everyone who keeps hens seriously uses hotline electric netting. Those who rely on chicken wire don't keep their hens long and consequently give up.
An energiser, batteries(get 2, have one on charge, the other in use)and 200 meters of fence will set you back £300 ish, but thats only the cost of 50 hens, and you have to pay only once, whereas once a fox finds a soft target they tend to keep coming back.
Someone pointed out to me that foxes will jump up, but they won't jump over things. Police dogs have to be specially trained to jump over obstacles, so they might be right.