Hens like to roost in trees. The higher they are the safer they feel. Are your roosting perches high enough off the ground Dan?
We inherited loads of hens here who all roosted in the trees, sitting like big fat puddings at dusk, as high up as they could get, swaying around in the wind, getting covered in thick snow and frost in winter. They have almost all lived to 7years plus. We used to grab them and put them in the hen houses in winter but they'd see us coming and fly up to the trees, eventually we just left them and they lived long and happy. We 'defrost' them in the mornings with porridge or rice crispies with warm milk which they all devour, Scottish winters.
They teach their chicks to go up too, we made a little ladder for them, they sit under their mummy's wings on the branches all cosy.
I'm in favour of letting them do what they want - we have houses, some use them, some don't and go in the trees. It stops disease and I think they are safer in the trees. Don't worry about their health, they're hardy wee things, stop being a soppy towny Dan
so long as they're up high. If you don't have a tree in their run, maybe get a windfall one from somewhere and strap it to your run fencing and let them roost in it?
They're happy if you're still getting eggs remember