My combi boiler is sat at 1 bar when it is not being used. when the heating kicks in it drops to 0.5 bar?

the boiler sits happily at 1 bar when my boiler is not being used however when i run a hot tap or the central heating starts it drops to 0.5 bar. the boiler will then turn on and off when the central heating is on. do i need to fill up the boiler with water? there doesnt seem to be any leaks around the system. the boiler is a potterton combi boiler.