Not to well chicken, potentially infectious coryza

[Jen1989]

If the doxycycline helped, and she has bubbles in her eyes, that points to mycoplasma gallisepticum or MG. It is always best to get testing or a necropsy if she dies, so that you know exactly what you are dealing with. As @dawg 53 said, there can be 2 diseases at the same time. Coryza is said to smell terrible, and sulfa antibiotics are useful to treat it. I would offer water often with a small lid or scoop help up to her beak. Keep her warm.

In the UK, Harkers Coxoid is usally available for treatment of coccidiosis.

Thanks, I've ordered Harkers Coxoid for the coccidiosis treatment. Managed to get her to a vet today, they think it might be MG, shes back at home indoors for the night with doxycycline and anti inflammatories till we get access to tylon tomorrow.
Vets have recommended putting all three birds onto tylon... I havent had a chance to speak with them in detail yet, this was a recommendation they sent over by email after I'd seen them in person earlier today. Does that sound like normal practice to you? I'm slightly concerned about overloading the two younger birds with antibiotics, one has been on doxycycline for the past week because of the secondary infection she got, the other I've seen sneeze once, shes a silkie, doesnt really get stressed, and I havent seen her show many symptoms other than both the young birds having had a reduced appetite. Either way, I'll buy the tylon so I have some, I'm wondering if I should do as they suggest and give it to all three birds...
I'll also be finding out for sure what this is if penny doesnt make it.

 

[All4Eggz]

Good luck !

 

[Jen1989]

Make sure she gets water; hydration is important

I have been, managed to get her to drink some by syringe earlier today and got her to a vets, we have syringe formula feed that's supposed to cover her food and water needs, hopefully it helps her to recover.

 

[Jen1989]

Thank you all so much, update on Penny, shes been to a vet today, suspected MG, shes deteriorating quite quickly, I'm still hopeful but preparing for the worst. Vet said she doesnt have a temperature, which is something, I've also been given formula feed to give her food and water by syringe, just waiting to see how her body responds to it (waiting for a poop basically). Only other update to share is that shes gained some fluid around the front of her abdomen, vet said possibly pneumonia, looks like symptoms I've read to be water belly...
If she doesnt make it, I'll request a necropsy for the sake of the other two birds, I'm hoping that's something that's as readily available in the UK as it sounds to be in the US.

 

[Jen1989]

Also forgot to say, no obvious stench on her, thanks to those who suggested to look for that, it's what's ruled out coryza and lead the vets to believe its MG.