Is she showing any other symptoms? Coughing, sneezing, discharge?
Do you know if they had been vaccinated before you bought them?
It is possible that Hetty may have a respiratory infection, if she had her eyes shut and was mouth breathing
There are a few different agents that can cause repiratory disease in chickens, some viral, some bacterial. A course of antibiotic may often be helpful though, as birds are prone to secondary infection.
The next link goes into more detail, explaining respiratory infection http://www.chickenvet.co.uk/general-health-and-common-diseases/respiratory-disease/index.aspx
I would isolate Hetty for now, place her in a comfortable box, lined with towels where she has easy access to drinking water and tempt her to eat with her favourite foods
There are some good tips on nursing poorly chickens on this link http://www.brittonclouse.com/chickenrunrescue/CARING%20FOR%20A%20SICK%20OR%20INJ%20%209_08.pdf
It would be a good idea to add aplle cider vinegar to the drinking water, which acts as a tonic http://poultrykeeper.com/common-articles-to-all-poultry/general/apple-cider-vinegar.html
I would also add in some probiotic or live yoghurt to the diet, to help boost the guts friendly bacteria http://www.chickenvet.co.uk/shop/products/index.aspx/category/nutritional-supplements-5
poultry friendly vets, can be found using the next link http://poultrykeeper.com/poultry-vets-uk/poultry-veterinary-practices-services-uk/
please reply if you have any questions
I'm so pleased to hear that Hetty is better.
Best wishes Fiona