Vaillant aquaPLUS filling loop
Turns out there is no flexible pipe, I guess I was reading the wrong manual. The system can be refilled by simply turning the screw at the bottom, as shown in the pictures.

The boiler needs to be powered off, and the screw (valve) slowly opened while keeping eye on the pressure gauge. Refill slowly to 1-1.5 bar (everything below 2 bar should be fine).

In hindsight, this seems obvious as the screw is right at the connection between the cold water mains and the central heating return pipe:



There is no need to remove the bottom grill either: