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).
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