Very hot weather, Cream Legbar hen suddenly lethargic and losing balance

[OwO]

I live in South East UK and it is very hot for us at the moment at 30°C (85°F) and one of my Cream Legbar hens has stopped eating or drinking much today after sitting down for a while under the trees. She walked very slowly back up to the run and I thought she was struggling with the heat or something, so I dipped her in cool water but I can't see a difference so far. She was fine yesterday and has been laying nearly daily until now. She is now standing in the run and will occasionally act as if she's about to tip over. What could be wrong?

 

[OwO]

Ugh I meant to post this in emergencies and diseases...