Boiler Combi 24 pressure indicator always point to 0
Take a close look at the needle of the meter:



There's a pin at about 7 o'clock on the dial that the needle would rest against if it were reading zero. But, the needle isn't against that pin; it either jumped the pin and is reading about negative 0.5 bar (pretty unlikely), or it's actually beyond the top of the range and reading about 4.8 bar (more likely).

Looking at the manual for your heater, and at the diagram for the connections at the bottom of your heater, that meter is on the heating circuit. The manual gives a maximum operating pressure of 2.5 bar (the safety valve is set to go at 3 bar). If the meter is accurate, then the safety valve has failed and your heating circuit is running at dangerous pressures. It's actually more likely that the meter is broken, and the needle is just hanging straight down due to gravity. Hard to tell without more information.