How should I increase pressure in a Glow Worm Micron 50FF?
The red knob might be for setting a pressure relief valve rather than for a filling loop valve.

I have the same boiler on a pressurised loop, it is pressurised as needed by opening an adjacent quarter-turn ball-valve (an "appliance valve", with a short blue lever) permanently plumbed to cold mains supply.

Your setup may differ, I'd try to find how mains supply is connected, it may need you to connect a temporary hose somewhere.