Why does my combi boiler keeps losing pressure?

my combi boiler works fine at heating my water and filling radiators but every couple of weeks the pressure gauge goes down to 0 and i have to fill it up again. I can't find leaks on the radiators ( also i live in an upstairs flat so i would have the women downstairs knocking if i did). I must admit i haven't had the thing serviced for some time. also how much roughly does it cost to service a combi boiler?