One of my hens is eating and drinking but is not well, and hasn't been for a week or two. Not at death's door, just not well either.
The others in her run are fine, never had a thing wrong with them. They eat layers pellets and some mixed corn and greens, and there is poultry spice in their feed. They have grit and oyster shell, and there's a supplement in their water to improve shell quality. Bedding, run, water & feed are clean.
It's just one of them that stands there, all hunched up and clearly unwell though not that bad that I expect her to die any moment. They've been wormed (inc the sick one), have no mites or lice, eyes and noses clear, tails upright etc. No penguin stands or problems walking. No blocked crop or foul smells from either end. Feathers at the vent clean.
I first noticed a week or two ago that she started having less interest in treats - whilst the others savaged the cabbage, she seemed to be not that bothered. She stopped laying, but then again she's an ex-batt whose eggs already had thin shells anyway regardless of all the oyster shell and supplements, and some of the other ex-batts had already stopped laying (except one brave little soul who produces an egg daily regardless what might happen around her).
Then she got worse a good week ago, one day I came to the run to find her sitting there on her own, looking miserable. I thought there might have been a link with the jar of earthworms a child had brought them as a treat a few days earlier (the chickens were wormed in September, but to be honest they did get some other food so the worming might not have been done properly), or some other gut-related issue as there was lots of diarrhea to clean up. So I wormed them all just in case and gave them some yoghurt daily as well but no other treats. That seemed to have done the trick as the pooh improved straight away, the ill chicken was better within days and seemed perfectly fine again a week later, just a bit less energetic than the others.
Phew, problem solved. Or not?
Yesterday she was a bit quiet again and went to be early.
This morning she was sitting all hunched up. Might it be that because the temperatures dropped, whatever ilness she was fighting off is now back? I'm worried about the cold, as the weather forecast for the coming week is -7, and the run is in a windy field (though with lots of plastic windbreaks around their run, inc a porch to the coop). When I read up last week about what could have been wrong with her, it was often mentioned to keep an ill chicken warm. Now I'm wondering if perhaps I should take her home with me instead of leaving her out on the field. She currently shares a coop with 3 others. At home she'd be inside in a balmy house so no energy spent on keeping warm, but she'd be on her own (well, apart from me

). No dogs or other pets in the house, just warmth and her choice of daytime television.
So what do I do?
1) Whatever this sick hen has wrong with her, the others aren't catching it.
2) She's a pet chicken, but I'm a bit reluctant to pay a vet (the appointment alone is £30) if they can't do much more than what I'm already doing. I kind of expect the vet to give her antibiotics and hope it'll sort the problem (whatever the problem may be). Taking her to the vet would also be stressful for her, she doesn't like being picked up.
3) Would being away from the others in an unknown but warm environment for a week or more be more wearing on her than being in her usual company but outside in the cold?

We've had chickens in our house before (long story, lots of disinfectant) so I'm aware of how much they pooh and she won't be allowed on the sofa
What would you do? Vet? Home and vet? Nothing?