Potterton Performa 24?

I need to bleed my rad's and believe this will result in a drop in system pressure which I need to rectify by adding water via the filler loop on the boiler.


I can see a flexible hose connected to the underside of the boiler which I am assuming is the filler loop. What is the exact procedure to add water? The loop I am looking at is connected to the system with one point of the connection having some kind if tap on it (if I turn this even slightly counter-clockwise something seems to start flowing)

I dont want to start mucking around with this until I am sure I know what to do !

Thx in advance