I`m not sure whether the householder is supposed to do it but underneath my combi boiler there is a sort of metal wovan hose and where that attaches to the pipe, there is a lettle black tap/lever. If my water pressure drops, I turn the lever and that lets more water into the system. The pressure should be between the 1 and 2 bar mark. The water can be heard coming into the system and when the bar gets to the correct level, I close the tap again. I don`t think it means you have a leak, it`s probably air getting into the system somewhere.