Help with worcester combi boiler?

I have a worcester 240 combi boiler it's about 12 yr old. Every so often the pressure drops and i have to top up the water level i have a valve under sink that lets water through, i tried to top it up today the valve was open for ages but the needle didnt move it's at 1 1/2 bar and the hot water upstairs is at a dribble and my shower indicates low pressure. If it was low pressure surley the needle would be lower than 1 1/2 bar and turning the valve on should make water level higher--help please