Your cylinder has two immersion heaters, one at the bottom of the tank and the other about 2/3rds of the way up.
The lower heater is designed to work on Economy 7 (or similar) overnight tariffs. The electricity supply to these is only available overnight, say 23:00 to 05:00, although your electricity supplier will be able to tell you the exact hours. Within this period, you can set the controller to determine the hours (e.g. 23:30 to 02:30) when you want the water heated. This immersion will heat up the whole tank, and they are well insulated so that water not drawn off will only lose its heat fairly slowly. Can't tell the size of cylinder you have from the photograph, but it looks like a 210litre one. You should get two baths out of this with some water to spare, although it depends on how deep and how hot you like the water!
The upper heater will only heat the top 1/3rd of the tank, and runs of normal rate electricity, so costs 2 or 3 times as much to run as the Economy 7. The most effective way to use the boost is not at all, but regard it as an emergency supply.
You need to check that there is electricity connected to the bottom heater during the prescribed hours, and that the lower heating element is working. Unless you are fully conversant with electricity and have a multi-meter and know how to use it, the only way to check is to:
1. Turn off all other electrical devices and storage heaters overnight.
2. Make sure the water is cool, then take the cheap rate meter reading at night and again in the morning.
If the reading has increased, that power has gone into heating up the lower immersion.
If you can find the make and model of the control unit, you should be able to find the operating instructions on-line.
