Worcester 24i RSF Junior boiler - pressure problem?

The pressure on my boiler keeps dropping to ½ or below and when the heating is on it goes up to 4. Sometimes the fault light comes on when the pressure is low and I try to switch the heating on or try to use the hot water tap. I have to top up the boiler most days and even then sometimes the fault light comes on. I paid an engineer £60 to fix it but he only replaced the air pressure switch which has not made any difference at all. Apart from topping up the expansion vessel has anyone got any suggestions?