Always great to see your words under my posts! Many thanks for lending your wisdom & experience to this project.
I was told that foxes can dig to a depth of 50cm and didn't fancy the task of putting the metal grate in vertically, so the horizontal idea is basically down to my laziness. However, i let it go because i find it hard to believe that a fox would be able to dig under from a distance in a single night. And even if it did, then what? The chickens will be safely locked up in their house. As long as i check integrity every morning there will be no easy access for foxes during the periods of the day when we are not there.
Rotating chicken paddocks sounds like an excellent idea. We have three pieces of land on which we could do this but how on earth does one lead chickens from one piece of land to another? A distance of around 100m. I suppose we could carry them if they will let us?