Thank you for your reply.
I have spent the day googling & reading forum post and I now think she may have a slipped Achilles' tendon. I have managed to straighten the leg and keep it stable. I have also separated her from the others so that she can get to her food & water without being pushed...
We've just had a new batch of 6 chicks who are almost 2 weeks old, but I'm starting to get quite worried about one of them. She has somehow injured her leg since being in the brooder and hops around on her one good foot. She gets around fine and will hop back and forth to the feeder and water...