Traditional creosote is a very good long term solution to red mite. I had the most severe infestation in my huts last year until I creosoted. It has to be the genuine crosote not the subsitute. This can be very difficult to get hold of for some people as its restricted.
Creosote is the best solution but if you can't track down that this is easier to get hold of -
Wood worm treatment. It's already to go, use a sprayer such as this -
DRAPER 48223 1.25 LITRE PRESSURE SPRAYER - Compare Prices and Buy at PriceGrabber
Then I used this stuff -
Woodworm Killer 5L from PPC Supplies
A 5L tin did one 20ft long by 6ft wide shed with some to spare.
There are some importatnt rules to follow when using this -
Hut is well ventilated, you don't want to be breathing this stuff in.
If it can harm you by drinking/inhaling it can harm your chooks too. Shut out your poultry for an hour or so.
Remove all drinkers/feeders & bedding before you start.
Treat all the hut particularly corners/crevices. If perches are removable spray into where they fit in and spray perches too.
When I used this, I had dead red mite in heaps within an hour.