removing wall vent cover help

[numpty2010]

hi i have a marley plastic adjustable ventilation grille which covers the internal wall hole to the outside grille, but i can't remember how to pop the sliding cover off the back plate. i can't see any marley ones online although manrose look like they employ the same type of design. the top cover slides back and forth to cover or open the vents behind on the back plate which is screwed to the wall. i think there are 4 slide through hooks but i can't seem to get it off or know which position the top cover should be in to release it.

anyone got one or know what i'm remotely on about?? image included below.
appreciate any help thanks

 

[flameport]

Slide to the left
Get hold of the top left and bottom left corners and pull them away from the wall
Slide to the right while keeping the corners away from the wall, and it should go all the way and come off.

If not, try the opposite - slide to the right first etc.