My guess is that your immersion is still on, despite you thinking its turned off. Presumably you aren't having a shower at midnight, so something is drawing 1kWh from 00:00 - 01:00.
FYI, even when I'm using energy intensive white goods my hourly draw very rarely goes over 1kWh in any hour period. The only time it exceeds that is if I'm running the oven, dishwasher or washing machine at the same time as each other
[Edit] even your overnight baseload is very high. You're using 800-900Wh in the middle of the night. For comparison, mine is 90-100Wh when we're all sleeping.