thanks all. they have been wormed regularly, so that's not an issue. I am in the UK so all they offer is Red Mite Kill, which is Diatom. I will try the links, thank you so much for your help.
I have 14 chickens, unfortunately we recently got red mites in the coop. Coop has been power washed, smothered in Diatom, and so have the chickens. everyone seemed so much bett,er egg production went up etc. Bar one chicken, she seems to be eating every now and again, she'll scratch around...