Represurise Vaillant Turbomax gas boiler – turning filling screws, but won’t fill

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

If you just shut off the gas without shutting off the electrical, it shouldn't damage the system. Basically the thermostat(s) will continue to call for heat, the boiler will try to fire up and fail, then the boiler should give up and enter lockout. The boiler has safeguards built in to handle a lack of gas, which should prevent damage to the system.