How to refill water into a valliant turbomax boiler?

Hi my daughter is staying in a student house. Her room on the top floor has a radiator which is warm at the bottom and cold on top. Water pressure on the boiler is 0.8 and so I think needs topping up. I have a diagram to show how to fill it but It doesn't seem to correspond to the boiler she has. There is a silver flexible hose underneath which has a black plastic screw top. I have unscrewed this but nothing happens. There is no key. I can't find anything else which looks like a tap to fill the system. Can anyone help?