0.4 bar and red light on boiler. Which valves do I open, what do they look like and where are they?

I have a potterton boiler. A red light is flashing saying 0.4 bar which I ume means low pressure. How do I raise the pressure? I know I have to open some valves somewhere but I'm not sure what these valves look like and where they are. There are no leaks anywhere I have checked.