Thanks Foxy.
Have been on the phone to a nice man ("very very nice man") who gave me all sorts of advice and is sending a couple of sachets of Fican W which can be made up for a spray. He reckons that as I've only got 3 hens it's very expensive to get Milbenex (I know....I got the name wrong!!) and also, as it's the strongest stuff you can get it's better to try this first. Don't want the mites getting immune too early!
Something else of interest...he reckons we must have a mite problem in Southampton because he has had several orders from here.
Anyway, thanks again. (I'm seriously thinking of saving for an eglu.....
)