So as I said I put my 3kw immersion on at 10.30 last night, didn’t take a reading of the temperature of the water but it was a cold weekend and was -1 last night when I plugged it in so water would have been really cold, Unplugged it at 8 this morning, drove to first job so the water had a chance to slosh around, took temp reading, temp of water at top was 45 temp at bottom of tank was 42 , that is approx 500 litres of water in a 650 litre tank. I was left with approx 150 litres at 2.45, temp of water was 29 ( no inslation used ). Will take another reading tonight and tomorrow and report the results.