My Alpha CB24X has lost pressure what shall i do?

My gas combination boiler has just stopped working. First the hot water stopped and the hot taps ran cold. The pressure gauge was in the middle of the green zone and no neon’s were illuminated. During this time the Heating worked fine. Then days later the heating ceased to come on and upon inspection the neon’s A and B were flashing Alternately, which signifies that the pressure is to low and the system needs to be re-pressurised. Sure enough the pressure gauge had fell to 0.5 Bar, well below the green zone. The only external control is the reset and this does nothing to change the situation.

Is it possible to re-pressurise the system myself? It has not been serviced since we moved in 18 months ago. Shall I call the Alpha help line or British gas? If any one knows what course of action to take or what might be wrong with it then I will be better equipped to avoid the rouge tradesman.