Oh dear, as I said, the British summer is perfect for them this year, sadly.
Duramitex Plus is definitely the way to go.
Until you can get your hands on it, you can use any insecticide spray that contains duramethrin or permethrin. Wilkinson hardware stores do a great own-brand one called 'Ant and Crawling Insect Killer' (it will only kill those mites that are live at the time of application though, so the new hatchlings will still come back a few days later), or even better, there's a spray called 'Dethlac', which effectively works in the same way as Duramitex Plus, by coating the surface with a durable oily lacquer of pesticide which kills all the live mites, plus gets the ones that hatch a few days later too, thus breaking the life cycle. It claims to be 'effective for months', so would be worth a go in an emergency, but I personally wouldn't rely on anything for a complete cure except Duramitex Plus.
Don't forget to use Creocote too. The mites will relocate to the grass/ground surrounding the coop, waiting for a chance to reinfest the housing unless you make it unappealing to them. Coating the surfaces and filling in all the cracks with creocote means they'll have nowhere to hide.
