Empty out the hen house and give it a thorough clean.
Make up a solution of dettol (about 1 - 2 parts dettol to 8 parts water)
Spray every nook and cranny of the hen house, make sure you remove perches etc if you can.
If it is a serious infestation, you will need to repeat this every 2-3 days for 2 weeks (they have the same life cycle as head lice, so you need to re-treat to catch emerging baby mites)
There is an ingredient in dettol that dizzolves the mites waxy outer layer, and kills them. (and dettol is much, much cheaper than most of the other products available :) ) Jeyes fluid does the same thing, but is poisonous to chickens, so needs plenty of time to dry.
We clean out every week and give a spray round of dettol every week, and (touch wood) haven't had an outbreak since.
Next spring, you could always contact a local fencer and get some proper creosote to paint the house with - inside and out, red mites hate creosote!
FWIW, plastic houses can still get red mite infestations, under the perches and in the nest boxes.