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Hi, Welcome to JustAnswer. You're through to Chris, a Vet based in the UK. I am reviewing your question and will get back to you as soon as possible
Sorry to hear of this with Flapjack, it does sound like she is very poorly indeed, possibly with a severe respiratory infection. Unfortunately once chickens get to this stage, invariably they do not make it. It may be better to humanely euthanase her to prevent further suffering if this is something you are comfortable doing, or get your vet to do so. Treatment could be attempted with antibiotics but I feel that her prognosis is likely to be guarded at best.
If you would like to have her examined first had by a vet then I would suggest checking here https://www.chickenvet.co.uk/find-a-vet
I'm sorry to not be able to give you a more positive outlook, please let me know if there is anything further I can do.
In addition if you have other chickens then it may be sensible to consider sending for a postmortum to establish to cause of her illness to help prevent the same problems with others in the flock. There is also a small possibility of this being Avian Influenza which would need to be reported to the AHPLA (DEFRA) as there has been a recent outbreak amongst poultry at several sites in the UK
You're welcome. Best wishes.