Potterton Powermax 155x losing pressure?

I have a powermax 155x and recently had a new Varem pressure vessel fitted however it keeps losing pressure over about a week it goes from 1.0 bar to 0.3 bar and I have to keep topping up the system with water. The radiators are not leaking and I cannot see water on the floor anywhere. Has anyone any idea of the problem or what it may be and to get it fixed please?