We had a major attack the other day in a 10 x 12 house. It was disgusting and DE does not seem to have helped this year. I tried to get some poultry shield but couldn't locally and couldn't wait for online to arrive so I bought a can of Nettex stuff. I don't usually like chemically stuff but I opened the shed right up and sprayed some around sparingly once the birds had finished laying around 4pm. By the time they went to bed there was no smell at all. I did this a couple times and a week on, all the mites have gone and the hens seem fine. Egg numbers are well up and the birds seem much happier. I have to admit that although I didn't like the idea of it, the mites were doing far more harm than a small exposure to the chemical in my view (hope). I also had an infestation in our small Sussex house but this time I sprayed it and shut the hens out ntil bed time. Again, killed the lot where DE had failed. I've noticed a dramatic improvement in shell quality with them as well, which is good news.
I'm not a fan of pesticides but in this case, we had it so bad, I felt I had to take urgent action for welfare if nothing else. I will get some Poultry Shield concentrate and give that a go though. I believe you can mix it yourself quite cheaply and a weekly soaking with a spray bottle might do as good a job as the horrible stuff.