Cant add pressure to system

Hi
Have an Electramate 2000 in my flat, I used to be able to add pressure to the system by turning the little tap inside the cover, as i tended to turn the whole system off when i didnt need it.

Had a Gledhill engineer come and service the system but he did not pressurise the tank, meter reads under 1 bar, so i tried to add pressure by turning the tap but nothing happens now, any ideas what has happened? Has he isolated something?

Help would be much appreciated

Thanks

Get the engineer back.
Or at least talk to him on the phone.
Mike