Sorry 
We fitted a water butt in our back garden this summer and we were wondering what we need to do to protect it over winter and through possible freezing temperatures. Do you empty yours, leave the tap open or what? Or leave it full and assume/hope it might not freeze?

We fitted a water butt in our back garden this summer and we were wondering what we need to do to protect it over winter and through possible freezing temperatures. Do you empty yours, leave the tap open or what? Or leave it full and assume/hope it might not freeze?
-30⁰C 