Thankyou all for your replies. I have made a note of all suggestions for future use incase I need to use them.
I decided to go for the ant powder option first as suggested by Clansman but didn't think I'd be that lucky. However, one week later, I have blitzed the henhouses, brushing the powder hard into all the crevices and joints inside and outside the 2 and rubbed it into the girls, liberally sprinkled on bedding as well. Added diatoms (which I always use anyway) just for good measure....not a sign of red mite since!
I couldn't quite believe it, so I repeated the whole procedure again after 4 days and so far, so good. Will watch like a hawk though. Hens all happy and bright. Laying as well as can be expected considering that they are moulting. Thankyou Clansman! And everyone else for being so helpful.