Tower block - how to bring the radiator loop up to pressure?

I live in a tower block (UK) which has a centralised boiler system. I believe (but not certain) the way it works is very hot water in some central block-wide closed loop passes through a heat exchanger and so heats the hot water that my flat uses for hot water and, also the flat's own heating loop (water radiators). The radiators have not been performing well recently. I had a look in the plumbing closet, picture at https://imgur.com/w8G0B42 , and there's a big pressure gauge front and center, at 0 bar. I'm guessing that that's the pressure of the flat radiator loop, and it's too low, and it needs some water fed into it from the mains water supply to bring it up to the red marker at 1 bar. So my question is... how do I do that? In everywhere I've lived before there's been an obvious valve that I can open to feed water into the heating loop. Here there are two obvious valves, marked 1 and 2 in the diagram. Both are closed. I've tried (cautiously) opening both for brief periods -- but neither makes the slightest bit of difference to the radiator system pressure. (There's also a red knob to the right of the gauge, with an anticlockwise arrow labelled '3 bar'. I can turn it to make a click which rotates the knob 90 degrees each time, but doesn't seem to actually do anything useful) What am I missing?