Chicken breathing with beak open.

Yes, it's the heat.  Make sure they have plenty of shade to rest in.

I certainly has been!

Make sure their water is in the shade too.

Mine too

As we had a good result with the eggs I gave them some watermelon, and cucumber. Think I am now their best friend

You are getting our normal summer temperatures and yes, our girls (and boys) always have their beaks open and wings slightly away from their bodies, even when they are roosting for the night. We found it a bit alarming our first summer, but they cope and it gets into the forties sometimes.

We give them all the ' watery'  veg we don't want and they eat it or not, whatever they fancy. In the heat it soon breaks down if they don't eat it. There is no let-up in eggs and no change in poos. The ducks need their pond cooling with well water during the afternoon and the area under the wall gets a soaking for the hens to cool down once the sun has moved over. Masses of shade from wall, walnut and pear trees and a stand of self sown sunflowers, and weeds, which they love (I know they are lucky to have so much space. ) Water needs to be kept out of the sun as it gets too warm, but the ducks need a top up of the mess they make of their drinking water as they need it to eat so it tends to be out in the pen getting warm.