The hens have only been in the tunnel since early December when the restrictions were put in place, so fresh chicken droppings are a new thing. It's quite a large tunnel and only 8 birds, and we will turn it in well, but I'll watch out for burning of foliage etc in the summer.
Normally we use well rotted sheep manure plus straw bedding from outdoor sheep shelters in the tunnel. It's composted for a year, then worked in with a mantis, but it has been suggested above that that is disturbing the soil structure. It does cause a slight pan, but we are used to that so break it up regularly, and anyway our soil is well drained.
Outside we use well rotted chicken house cleanings, again with straw, or we lay it fairly fresh on the ground, cover with a tarpaulin and leave for a year, by which time it's been broken up and taken down into the earth by worms. Everything is ok outside, except that like you only the hardiest stuff grows outside here - too windy and cold. That's why I was stuffing the tunnel with plants, but now I will grow less and allow more space and air for each plant.
I looked up soil testing kits and lab services and the useful ones which do trace elements are expensive. pH and N,P,K testing kits don't get good write-ups for being inaccurate. I'll investigate RHS a bit more - can't remember if they do trace elements too. I have to decide if the cost of a good lab service is worth it, or if I should just continue trying out possibilities and see how well each one works.
Mr F has decided that today we will spread a thick layer of molehill soil over the whole tunnel and let the hens work it in, and pick out any weed seeds. Hopefully that will reintroduce enough soil which hasn't been exhausted by growing in such a restricted space for 21 years. It will also introduce stones, which I'll spend the next few years picking out....