Karen, I'm sorry that your question wasn't answered in a timely manner. We don't have many avian vets on this site. Unfortunately, there's quite a problem list - stopping of laying, polydipsia (increased thirst), inappetence, somnolence (sleepiness), diarrhea, and cyanosis of her comb (lack of oxygen to her tissues) and mites which can indicate any number of illnesses or health issues in chickens. In avian medicine, there's rarely one cause of such a presentation, so we usually begin with a list of differential diagnoses and use lab tests, X-rays, and physical exams to differentiate one from another. Necropsy of a newly dead or a sacrificed severely ill bird then refrigerated (not frozen) can be an important diagnostic particularly in large flocks. With this in mind, your best course of action is to reach out to your county-extension poultry personnel or avian-oriented veterinarian for help. Please see here: www.bvpa.org.uk
It's best to approach the diagnostic process with a clear sense of Mary's financial value to your operation. Although some services such as your county animal disease diagnostic laboratory might be available free of charge through a county agency or land-grant extension office, the expense of some diagnostic tests and treatments can add up quickly. While it’s always worth your time and money to identify a bacterial or viral infection that could potentially impact more than one member of the flock, this might not be the case with a condition that only affects one hen. It frustrates me that I can't be more specific for you but such is the dilemma of poultry owners and vets alike.
What's particullarly frustrating is that in the UK you're no longer able to purchase drugs in order to medicate your own chickens. A vet has to prescribe these drugs. I might presumptively worm to remove roundworms and treat for coccidiosis considering her diarrhea and I would initiate a broad spectrum antibiotic for what is likely to be widespread bacterial infection in Mary.
Please respond with further questions or concerns if you wish.