re-pressurise

Hope someone can help. My old Worcester 24i junior combi boiler stopped heating water this morning (red error light flashing slowly). Noticed pressure had dropped below 1 bar so turned little tap by the loop/hose to let some more water in to the boiler. Have done this before and this usually...

This question has come up before in this forum but my setup seems different. Can anyone please help? How do I go about repressurising my C24? Images attached. Thank you.

All the black taps look to be open but they are all really hard to move so I don't want to move the wrong one. There is no flexi hose.