The unit in your kitchen is the boiler and looks like a very simple gas model with a simple temperature control. There should somewhere be a time clock to control it. The plumbing near the tank looks like you have radiators for room heating so there should also be a room thermostat somewhere.
The water in the tank is normally heated by the boiler. The two pipes that go around to the back of your tank will be fitted to a coil of pipe inside the tank, and the boiler circulates hot water through these to heat the water in the tank. The electric immersion heater is there because it can heat the water faster than the gas boiler, however it does cost more to run. Typical uses for this are:
- In an emergency if the gas boiler has broken down
- If you need a lot more hot water than normal e.g. several guests staying all wanting hot showers in quick succession.
In your photo there is a second switch on the same wall nearer the camera. This is probably the master switch for the system, it looks like it is on in the photo and should normally be on. The immersion heater switch you should only turn on when you want the extra heating power (and are willing to pay for it). If you hare having some work done, you could think about moving the switch outside the cupboard and changing to one with an indicator light so that you don't forget to turn it off once the extra need is over.
The timing of the boiler running should be mainly controlled by a time clock somewhere, but it will only run if there is need, that is if the cylinder thermostat (white cuboid on the front of the cylinder) indicates that the water in the tank is colder than it should be, or if the room thermostat indicates that the radiators should be on.