[Cornish man]

It could well be depluming mite use ivomectin drops on the neck.
Twice over 3 weeks.
Creosote your sheds too ! If that won’t work it’s exholt which is expensive but works.
A friend of mine had this problem and using the ivomectin on the neck it’s worked.

[fgaskell]

Thanks all for this very useful advice.
Have treated the henhouse thoroughly.

[chrismahon]

Please don't creosote  the coops and put the hens back in straight away. The stuff needs to dry and lose the odour- so at least three weeks.