Can't top up pressure of CH system

[Capirex]

We have a 'tribune unvented system', apparently.
The pressure is low (0.5 BAR) but I can't seem to be able to top it up.
If you look at the photo below, you can see the white top up valve.
Opening that does precisely nothing.
No sound of water, no movement in the gauge.
Any ideas?

 

[kevplumb]

tribune is an unvented cylinder

you are trying to top up system pressure on an unvented heating system

whats at the other end of the white flexi

 

[gblades]

Is there a non return valve at the end of that flexible pipe that connects to it. If it has a built in isolator make sure it is open.

 

[Capirex]

tribune is an unvented cylinder

you are trying to top up system pressure on an unvented heating system

whats at the other end of the white flexi

It's a solid (albeit bent) pipe, not a flexi hose.
It goes through a hole in the floor.

 

[Capirex]

I'll give some background, it might help.

1/ We had a new rad fitted in the kitchen, which has needed constant bleeding (once a week, approximately).
2/ A radiator in one of the bedrooms stopped working, it wouldn't bleed.
3/ Scottish gas engineer called and he identified low pressure.
4/ My wife doesn't have a clue what he did, but he did "something with the red bottle"
5/ Based on my experiences with other boilers, I tried to turn the top-up valve to get the pressure up to around 1-1.5 BAR, but nothing happens.

The heating appears to be working fine, I'm just concerned that the current pressure (0.5 BAR) is too low.