Well, after chasing another fox away earlier, I spotted my son's favourite hen with bubbles in her eyes. next time I looked they were clear, and then bubbly again.
I have moved her into the rabbit hutch (now vacated by the gold silkie) as I can't put her into the shed as the two broodies are there.
I have given her boiled egg and garlic in her water. Her tail is still up and she is still eating and drinking, and I can't hear any wheezing or other breathing problems. Any advise on what else I can do? Weather is shocking which I don't think is helping.