Advice regarding central heating pressure… (old system)

Hello,

This is not exactly an appliance question but maybe someone can offer some advice. I bled the radiators recently and now the pressure reading is 0 (it should be 1.5). As a result the radiators are not getting fully hot especially upstairs. I cannot remember how to increase the pressure again.

https://postimg.cc/N5kp2F08

In this pic you can see a small black switch or valve just under the grey washing machine hose. Is it this I use ?

or

https://postimg.cc/k25FVyz9

In this pic you can see the expansion vessel and pressure gauge. I was thinking it might be the red cap above the gauge that I use? If so how do I use it? Direction etc ?

I seem to remember it was one of these the heating guy used when I was shown along time ago.

I’d really appreciate any help you can offer.

Thanks very much.