Sounds like Mycoplasma to me as well WoodlandsDevon. Many Pedigree flocks carry it, including ours, but it remains suppressed if they are healthy and happy. We have only lost one bird to it due to us foolishly using ant powder to control red mite- it caused her immune system to collapse. If she was an immunised hybrid in a flock it could also be an IB flare-up.
Separation is a wise precaution as she may have recently contracted it from wild birds, then you need to work on giving her system a boost. She needs to be kept warm and dry. Try 0.5mL of Cod Liver Oil with multi-vitamins daily. Extra vitamin A in it will help the immune system. Vets prescribe Tylan in the drinking water, but I'm not keen on that.
If the symptoms subside you then have to decide whether to re-introduce her. If you have the facilities you could start a closed flock with any others that may show symptoms. Good luck with her.