Worcester 240 , trying to pressurise . ?

I am trying to pressurise my Worcester 240. It is currently sitting at 1/2 bar. The external filling loop is constantly connected ( we have only just moved in ! ). When I open the valves a little bit of water dribbles from the top of the check return valve but nothing else happens.

If I were to replace the filling loop ( depending on answers ) I assume I would have to turn all the water off at the mains supply?