Your cylinder has two immersion elements. The lower one is (should be) wired to your off-peak "economy 7" supply. The meter will have a timer in it to charge you cheap rate during those overnight hours, and quite likely will turn on the off-peak circuits (usually including storage heaters) only during those hours. If this is the case, leave the lower element turned on all the time. This will fully heat the cylinder overnight, and as soon as it reaches the preset temperature (might be in the region of 60C which is pretty hot) the thermostat will turn off the power so it uses no more electricity until the next night.
If the meter does not have a timeswitch, you can set your own to come on just after the off-peak period starts, and go off just before it ends. This is also a good time to use high-power appliances such as tumble drier, and to a much lesser extent, dishwasher and washing machine. You are advised not to use the tumbledrier when you are going to bed, in case it catches fire. This is rare, but not unknown.
The upper cylinder is (should be) off your normal full-price daytime electricity, and you should leave it turned off unless you have so many hot baths in a day that you use up all your stored water. In which case you can turn it on for a limited period. It will warm the water at about one litre per minute, so a bath would take about an hour and a half. Turn it off as soon as your needs are satisfied and wait for the cheap rate to top it up overnight.
Your switches might have red LEDs to show when current is available, but if not, you can tell when am immersion is using electricity, because if you listen intently, there is a slight "siinging" like a kettle. Or you can go and look at your electricity meter. It will flash must faster when you turn the immersion on.
If you post photos of your consumer unit(s) with the flaop open so I can read the labels, and of the meter(s), and any boxes around or connected to them, and the thick connecting cables, I can probably work out how yours operates.
Depending on age, you might have a "Teleswitch" (radio controlled timer) or a sort of electric clock with a visible dial. You can't (mustn't) touch the works or adjust them.
The exact time your off-peal starts varies frok house to house. This is deliberate to prevent twenty million houses all starting to draw high current at the same instant. But if you note the start and end time, yours will not vary by more than a minute in future unless something has gone wrong.
If you have a Smart Meter, the electricity fairies might turn it on and off as they see fit. This is probably not happening yet.