I can't get Prod's link to work, but the website for pestcontroldirect.co.uk does list quite a few products for control of red mite.
I've used Poultry Shield and it is very effective at killing an existing mite infestation, but you need a decent pressurised sprayer to get the diluted product into every nook and cranny. Most garden centres will have them and they don't cost the earth. It dries out the mites and you get a snow storm of dessicated little bodies a couple of days after use, so you know it has worked.
If it is just a preventative you want, most people use diatomaceous earth powder puffed into the joints and around the perches. Hillfooter (a forum member of old) used to recommend making this into a slurry and painting it onto the coop interior. He was a font of knowledge and this tip has been widely used by forum members ever since