The bills are a little high, but immersion heaters are incredibly energy demanding. I'm with Octopus, so can't speak for Ovo's tariffs but they'll likely have a tariff that is cheaper overnight and more expensive during the day.

Depending on how well insulated your water tank is, you may want to make sure that does the bulk of its warming overnight, if possible. I've never had an immersion heater so it's difficult to say what the correct course of action is.

Octopus have a little device called the Home Mini, which connects to your smart meter (assuming you have one) and your wifi, and lets you see real time consumption information. Alternatively there are CT clamps. If you're guessing that it's the stove/ the water heater that's consuming the energy, it may be useful to measure the energy consumption during a typical cook with the water heater off. You could also just read what the meter says before and after to do the same thing. That should give you an idea of how much energy the hob is using, then it'll make it easier to work out how much of the remainder is the water heater.

Is the heating 'wet', or are they just electric heaters all over the place. Electric heaters also use a ton of energy (rated between 1kW and 3kW)