Leaking CH system?

[Junior19]

I've recently moved in to an apartment which has a Potterton Suprima 50 Central Heating system.

Now the last few weeks the system has started performing poorly, with several lock-outs (red led flashing) and the pressure gauge now shows less than 0,5 PSI.

I found a small wet patch underneath one of the valves close to the pump.

When checking the screw on top of this thing it was very loose, and water came seeping out.

As I've never lived anywhere with a CH system before, I'm not sure if this one should be open or closed, if it is a pressure valve or a bleeding valve.

If anyone could help me shed some light on this I'd be most grateful.

The pressure gauge:

 

[Junior19]

I understand from the previous posts here that I would need to repressurize the system using the filling loop.

Is that the one I've found here?

 

[Breesey]

The thing in your first picture is an auto air vent. The black bit on top should be loose but it should not be leaking water. If it is replace it.

The braided hose in your third pic is indeed the filling loop.

 

[Junior19]

The thing in your first picture is an auto air vent. The black bit on top should be loose but it should not be leaking water. If it is replace it.

The braided hose in your third pic is indeed the filling loop.

Thanks.
I'll have the guy who installed it over for a look at that auto air vent.

It's leaking at a rate of approximately 1 litre every 6 hours now.