How to repressurise an alpha hecb25 boiler?

Hi- have v low pressure on the boiler and although the heating and hot water are working it frequently cuts out and needs resetting. Got a plumber coming Friday but he said need to adjust the taps at end of filing hose underneath. Can see this hose and have tried turning taps but nothing happening and can't hear water going in. Must be doing it wrong- anyone have any specific instructions on exactly how I do it please?
Thanks for any advice.