Can't find filling loop

I thought that myself. Opened it, nothing happened. I'm assuming I'm missing something incredibly obvious here?

It's currently sitting at around 0.5/0.6 bar.

The pipe with the 1/2 gate valve runs to the tank in the attic. I think the valve is simply to cut off water to the hot water tank if needed. There's an old feed and expansion tank in the attic which, I assume, is from the old system as the Logic 18IE is a sealed system.

I'll try tapping the pipe alright but I don't think its the filling loop (I also have zero idea what I am talking about!)

The blue line connecting to the 1/2 gate valve runs to the bottom of the tank. (i assume is just fills it up?). The red line is the coil return. The pipe from the attic (blue line) runs behind the yellow box and doesn't connect to it in any way. I don't see where the coil return and the supply from the tank interact.

The documentation for the boiler indicates the minimum operating pressure is 0.3 bar so I don't think it is a low-pressure sealed system.

Apologies if I am being dumb here, I'm just trying to top up my boiler