Headfry
Wasn't sure about introducing another one do they get on? Perhaps I could call it Flowerdew....

and Paul,
I've found the branches sticking out of the waterbutt together with the chunk of polystyrene floating on the top of the water keeps air available, And I'm on the allotment every other day so any ice that is there I lift off and stick in one of the other water butts.
Best wishes
Sue