You can carefully use a blowtorch in the hut at dusk on a daily basis for a week, preferably having moved the hens to another hut. Remember, the life cycle of red mite in warm weather is just 10 days from egg to breeding adult. If there is felt on the roof, remove this, clear away the mites and replace the felt with corrugated clear plastic on the roof boards.
You could spray the hut with a dairy insecticide, such as Deosect, but nothing will get in to all the cracks and crevices where the mites live in daylight – from where they crawl down the walls at night and along the perches until they reach the hens and suck their blood – therefore, if you have an infestation, treating the birds with a product that kills the mites when they feed will be more efficient.
There are no products licensed to treat hens against red mite which go on the bird. The best product I have found is Eprinex, a cattle pouron with no milk withdrawal. Your vet will ask you to sign a disclaimer as it is not licensed. Put five drops on the shoulder skin of each bird (do this at night, they stay still) using a laboratory pipette to keep the size of the drop standardised. That is for large fowl, bantams have three drops. You will need to repeat the treatment after four weeks so the mites are still killed when they bite the hens, and then every two months in the warm weather or when you see whitish powder on the perch ends, or tiny blood spots on egg shells. Do not eat the eggs for seven days after the treatment.
After the mites have gone, paint the ends of the perches with creosote, which does kill mites.
Do not paint the whole house with creosote, you would kill the hens as it is very toxic.
The old-fashioned method of using swinging perches, so the mites cannot access the hens, will be preventative but if you get another infestation, the mites will have to be controlled as they will reach any hens which do not perch, plus get on to the hens when they are laying in the nestbox.
Nestboxes are recommended to be set in the darkest place in the hen house, to encourage the hens to use them, but this means that the red mites like them as they are dark!