How can I fix my Ferroli combi boiler losing water pressure?

I have a Ferroli Moderna combi boiler and have to keep on re-pressurising the system (by using a filling loop) because it falls from 1.5 bar to virtually zero over a period of time (about a week). The pressure seems to be maintained after using hot water but not if the central heating has been used. I have checked, as much as possible, for leaks on the internal plumbing and also taken water meter readings which show that there are no leaks. When operating the pressure does range from 1.5 to about 2 bar (but generally no higher than this) with very little (if any) water being vented off. I believe that the answer may be to re-pressurise the expansion vessel but is this correct and how do I do it? Any advice would be welcome.