I don't know if the following will help. The Water heater should have two elements.
The lower one is the main element and should be connected to the economy 9 supply that comes on by supplier timings.
The upper element is the boost element and should be connected to the normal mains via a timer switch. It isn't beyond the realms of possibility that the elements have been swapped by mistake. Yon can check by following the cable from each switch to see if it goes into the side of the heater next to the element expected.
When I moved to RQ |I noticed my charges were a bit odd and the reason was that the electric company was recording the night reading as the day, and day as the night