Hi We have recently built a large run for our rescue battery hens. The area is approx 300msq and I have fenced to a height of approx 8 foot. We were recommended to use a flexible rabbit wire, so that foxes struggle to climb it. We have also surrounded the entire area with three strands of electric fencing to deter foxes. The rabbit wire has been laid along the ground, approx 1ft from the fence, again to deter digging. the wire has been secured into the grass and hopefully quite soon will be covered with grass to help keep it in place. Apparently if foxes or badgers try to dig under a fence they will try tight up to the fence line.
We do not shut our hens in a night and have taken this advise from local who built a very similar one 2 years ago and has not had a problem with foxes. I am keeping my fingers cross.