Thanks, those links gave great information.
It's coming to evening now and she's much the same as she was this morning, maybe a bit more wobbly but not too much worse. It doesn't seem to have progressed beyond that one leg, but the leg is definitely looking weaker. She is still able to stand and hobble, but she's choosing not to most of the time. She's spent a lot of the day either sitting still, sitting and preening or sitting and pecking the ground. She hasn't been scratching with her feet but she has been pecking into the soil and eating plenty of insects and grass etc. She seems alert and she stands up when we come over with food. She doesn't seem hugely keen on her normal pellets, although she has eaten some. She has eaten an apple, lots of cheese and some cake. We put her anti inflammatory on the cake to get her to take it. She's also eaten some oyster shell grit.
She's been in the shade all day. I don't know how to get her to drink more. She has had some water but not as much as usual, I think. I'm hoping she'll have got some hydration from the apple. I don't want to resort to forcing water into her unless I have to. We've given her some yoghurt and she's standing up eating that now. Hopefully that will give her some hydration, too.