Glowworm Fuelsaver Complheat 80

Hi, I hope someone can advise me... I've got a Glowworm Fuelsaver Complheat 80 Boiler - sealed system and it has been installed for about 6 years. I'm seeing a somewhat errratic fluctuation in pressure in the system (as shown by the pressure reading on the boiler) and have to quite frequently (every 2-3 weeks?) top up the system with water. There are no obvious leaks anywhere I can find.

Could this pressure drop be due to air venting or is there some other possble reason? Slow leak from valves that evaporates?

Any advice gratefully received. Thanks.

expansion vessel charge could be low. the waters probably leaving the system through the safety valve to outside

Following an earlier post think my expansion vessel charge could be low resulting in pressure loss. Does anyone know how to set this/correct this?

Thanks.