Have you got an Electramate 2000 thermal store? You should have a separate switching circuit which is run from a normal 13A supply. Forget the meter, that can be connected via a time switch so it activates at the E10 times and heats the water to 85C rather than 65C. If you look on page 10 here you’ll see “off peak signal supply” on the rhs of the diagram. That’s what you need to sort out.