How do I restore boiler pressure through the filling tap?
I just googled the manual for the glow worm series of boilers and it look like you need to connect a flexible hose from the domestic water supply (far left if looking at front of boiler to the bottom of the thread under the blue cap, and then turn the domestic supply on. with out the model number of your boiler it is a bit hard to give you exact instructions. but the manual does say that the hose should be disconnected after pressuring which is why its probably not straight forward just looking at it. try this page glow worm manuals