Sorry my mistake, It's for central heating. It's a oil boiler timer for central heating to heat all the radiators in the house. Once it's switched on we also get hot water from all the taps.
The immersion has a separate timer in the hot press, which we don't use. As we normal heat the water with the oil boiler. In the summer we close of all the radiators with one red lever in the hot press, which then heat up just water to give us hot water for shower.