Your birds may be suffering from a bacterial, viral, fungal, or parasitic problem, all of which need to be properly diagnosed by a vet. professional to be correctly treated. Although the initial outlay will be costly, if you wish to have and maintain a flock, it is sort of a necessity to get help until you have a feel for what's going on.
Here is a site for UK poultry vets: http://poultrykeeper.com/poultry-vets-uk/poultry-veterinary-practices-services-uk/.
If you do lose another one, it might be less costly to have a post-mortem exam performed to protect the rest of your flock.
Until you can get to a vet, please isolate all those who look sick - even mildly so - and put them in a secure, warmed clean environment with pure water, and lots of tasty treats, such as a mash, chopped greens, and juicy pieces of fruit (in addition to their regular food).
I hope this helps and answers your question. If you are satisfied with this answer, please press the green ACCEPT icon. If you have more questions about this problem, please press 'reply'. Bonuses and feedback are always appreciated. Thank-you.
Thank you for your well-intentioned advice, however I have to say that basically telling me to go to a vet kind of defeats the object of Just Answer. It's like an online mechanic telling me to go to a garage to sort my car problem out if you are with me.
Unfortunately I have now had to dispatch the chicken.
Dear Mr. Smith:
I am sorry that the purpose of this site was not made clear: it is purely an informational one. I cannot tell you what is wrong with your bird because I have never seen her; I can tell you my best guesses and offer you suggestions on what you should do next, but I can in no way, replace the need for an exam.
Please accept my sympathy for your loss,