Thanks for the link cowcreekgeek.
It's not that I don't trust the vet, it's just that I think she's more used to dealing with large flocks of birds kept in runs, whose only water source is the trough provided. Since mine have the whole back garden, and every blade of grass is soaking wet at the moment, they're just not drinking much out of their bowl.
I've found the solution though - I've mixed up 5g of powder (minimum amount my kitchen scales will measure) into 100ml of water, and I'm putting 20ml of this mixture a day (using a syringe to measure) into a bowl of porridge - not completely accurate, since I'm sure Cinders gobbles more than her fair share, but since she's the biggest and also suffering most with sneezes and wheezing, I'm not too worried. All the porridge disappears, so I know that around the required amount is getting to each hen. (1g of powder per day for 10kg of chickens - more or less!
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As a side note - Cinders laid 2 paper-thin-shell eggs today - together! She didn't even make it to the nest box - just laid them on the ground where she was eating. First time I've had an egg laid out of the nest box, and never 2 together, although the thin shell is an occasional feature of Cinders - she's only been laying a few weeks. I don't know if it's a side effect of the antibiotics, or just the fact that she still has a few kinks to work out; Has anybody had that before - literally two laid at the same time!!