My Valliant Turbo Max Boiler loses water pressure?

The F22 code kicked in last year, now I check the display regularly and top up by opening the valve. It is getting worse. I now top up every 1.5 weeks. Worse when the radiators are on . I cannot see a leak. Access to underfloor pipes is difficult so before I get under there, could it be something else?I am beginning to wonder if the pump has a fault,but would this help to lose pressure like this?