I'm sorry that you ended up with sick birds. I'm glad you are able to seek vet care.Thank you for the advice everyone I've had a read of all the information provided and her symptoms do seem to be consistent with CRD.
I will be taking her and all my other chickens to an avian vet soon. I've also inspected their coop and enclosure, there doesn't seem to be any sign of microbial growth but I will give it a good clean today.
Honey is fairly new to our flock, I contacted the seller for advice but they didn't respond. I spoke to my friend who also bought chickens from them and she said that her chickens have the same problem. Apparently she went back to the seller's property and found out that the chickens are bred and kept in a tiny, dirty cramped shed (with no windows) and are only taken out to free range before customers come. She has reported the seller to the council.
However I haven't seen this myself and I have no solid proof that Honey's condition is related.