Thanks for your replies all. I am wondering about Myco. The hen in question has recovered and looks well, as if she's just had a cold. Unfortunately, one of the others dropped dead suddenly on Saturday - she was fine, running about and eating well in the morning, by the evening she was fluffed, barely able to stand or keep her eyes open and despite best efforts (drenched chicken vits, honey and yogurt) she died shortly afterwards. The others all look fine, but I am wondering whether or not to go ahead with rehoming some more ex-batts in case there is Myco and they get infected.