I don't know if "The Landsman" is restricted to South Devon, I get mine at Mole Valley Farmers and it's free!
Pammy Riggs is a regular and very experienced contributor and I read this article by her yesterday,
To precis a bit, "Red mite do not live on the bird but lurk in the house and only go onto the bird when the warmth of the roosting hens lure them down."
To overcome this she recommends the following............"If your house is on it's last legs, burn it and get another" (an option not available to most of us, D.L.) or try this.
"Take one hen house, ensure enough material strength to suspend the perches from wires or strong string from points in the roof structure. For a small flock a cut down branch from the hedge is good enough. For a larger flock a grid- like perch gives stability. You have now foxed the red mite army! Any living in the structure of the building will have a job to find their way to the perches where their meal roosts. For a belt and braces approach a decent blob of axle grease or similar high up on the suspension wires gum up their tiny limbs.
Don't expect that to be an end to it because re mite will infiltrate anew. Keep a close eye out for telltale signs on the suspended perches and remove them to the bonfire. Re-string a clean set and off you go for another few months mite free. Use cunning, not chemicals and your birds will thrive."
You could possibly look the whole article up on Google.