Hi David, what a lot of relevant questions! I guess I over reacted. She's a Pekin bantam and she actually seems ok in herself - when I stop panicking. She's eating and drinking and hanging out with the others. Her vent does look sore, but I think the real problem is the fact that she seems to have lost most of the feathers from around her vent which rather draws attention to it. Do they usually moult in patches - could this be what's happening? Or maybe she or one of the others is pecking them? Anyway, I guess none of this is life threatening, and I 've calmed down now. I'm just so used to chickens going downhill so fast after first noticing that something's wrong.
Thanks very much for your voice of reason!