Thanks Ghost, she's actually been much better e.g. Drier, each day since I posted this question and we've had quite a bit of rain, so I think her condition is returning. She's certainly preening properly which must help.
I actually left the door to the polytunnel open by mistake on Saturday lunchtime (it was pouring down up here) and only remembered in the late afternoon. I went tearing up and found all six hens sleeping inside but on the old carpet we have by the door. Little chick was lovely and dry as you might imagine!