Immergas Boiler Low Pressure

I have an Immergas boiler which has been causing problems off and on. I have just noticed however that the pressure is down to 0.5. Anyone know how I can get the pressure back up? There doesn't appear to be any obvious dials, switches etc. Thanks in advance.

My system has a little tap which allows me to build back up the pressure. I imagine all plumbers fit something similar?

The pressure vessel may need to be adjusted.

The tap mentioned is for the filling loop, used to replace water lost from the system.