Boiler - Cant find filling loop to top up pressure

Anyone know where I can find the filling loop to top up the pressure.

Had an air lock in an towel radiator in the bathroom, bled the radiator and pressure dropped to zero. No idea how to top up the pressure back up now in the absence of any obvious filling loop?

Current Setup - Please see attached pictures

Boiler is located in back bedroom 3 valves in total - Black on the left, Yellow in the middle, Black on the right (Pressure on boiler gage reading zero)

Expansion Vessel located in attic - Two red valves (Pressure on expansion vessel gage reading zero)

Airing cupboard / cylinder tank located downstairs, couple of red valves

Have tried opening the 2 red valves on the Expansion Vessel but they don't seem to budge very far before there is resistance and I didn't want to force them because I'll be honest this is my first time I've ever had to do anything with boilers and radiators. Would be grateful if someone could advise?

Thank you