Thanks for the help and suggestions everyone.
Mark, your setup seems interesting. But I'm not sure I understand exactly what you are doing to get "In one year the seven hens on seventy square metres have added seventy five mm of magnificent fertile soil"..?
If you have a setup with 7 chickens giving extra fertile soil in just 70 square metres, then that is incredible, normally that many chickens in that much space would make soil so hot with their poop after a year, nothing would grow in it. Could you explain a bit more in detail please? I'm not sure if it's something to do with the horse manure or the corn you are growing?
For me, I still think the best option for the amount of space and chickens I have is to have small areas (about 1 metre square) around the yard with fast growing weeds like chickweed in one area, clover in another, comfrey in another and maybe some mixed weeds in another. Each area will be covered with wire mesh (low to the ground) so the chickens cannot get into that 1 metre square and completely kill that particular weed off, but they can eat whatever grows out of the chickenwire whether from the top or from the sides (and with a a good few square metre patches of fast growing weed, they should have a fair bit of munch on most days hopefully). As for the rest of the bare yard where nothing is growing (and really that should end up just being a lot of paths all around those square metres, fruit trees and shrubs), I think woodchip mulch is really my only option, I don't think any other groundcover can survive the chickens.