Well it was -7C here up the mountain last night.
Put the immersion heater on this morning as it was cloudy and then 'POP! - Water fountain squirting from the roof tank from a burst joint.
We'd put anti freeze in the solar panels but the actual water tank had huge blocks of ice in it that the repair man took delight in throwing onto the driveway.
As I write it's -1C and will be -6C later ........