Your immersion is 20kw? I thought they were 3kw max? 20kw would cost you £11 for 1 hour. that's got to be wrong.
Sorry i wasn't clear it's 20kwh for the whole house for the whole day. Of that, I think the immersion heater is responsible for about 5-6kwh of that. Friday's figures were 13kwh when I lowered it to 50c instead of 65C. And without doing anything different Saturday has shot up to 20Kwh again.