Found it this morning with its legs completely splayed (bad case of splay leg) and very weak since it hadn't been able to get to the food and water overnight. So I've cut a strip off the end of a spec-cleaning rag (SpecSavers, nice n soft) and tied its legs closer together, as per instructions on internet...
...and he/she is walking about now (with its funny little SpecSavers attachment, eating and drinking, and keeping up with the other chicks, bless it.
So pleased with myself. I really thought it was a lost cause first thing this morning.