1. It appears that your water is heated only by one or both of the immersion heaters.
2. As Rodders53 has said, the immersion heaters have the white circular covers held in place by a domed nut.
3. There will be mains electricity voltage inside these covers. Unless you are absolutely certain you know how to isolate the electricity and test that is is isolated, do NOT touch these covers.
4. It is usual (though not certain) that the lower immersion heater is connected to a cheap rate source of electricity (Economy 7 tariff or similar). This heats the entire cylinder full of water up on "cheap" electricity overnight.
5. The top immersion heater is normally connected to standard rate electricity, and is meant to offer a means of topping up the hot water should you run out.
6. A full cylinder of hot water:
6.1 Ought to offer a single occupant sufficient hot water for a couple of days.
6.2 Ought to remain hot for more than 24 hours. Those cylinders are very well insulated.
7. I'd suggest you trace the cable from the top immersion heater to its switch, and turn it off.
8. See how you then get on for hot water. If there is none, or very little after a day, the lower immersion heater may be switched off, may have tripped its overheat thermostat and need resetting, or may be faulty.
9. You shouldn't reduce the immersion thermostat settings below 60 degrees, as you would then run the risk of legionnella.