Dimmer switch dis-assembly

[swarfega]

Excuse my amateurness
Just bought a nice new dimmer switch that I plan to get installed.
It's a face-plate insert to screw into the wall, with a blank cover plate over it. Problem is - I can't seem to separate the two!
I've prized the faceplate away from the blank cover as far as it will go, but obviously the dimmer pokes through both and I cannot separate these. Please advise!

 

[Mr Mark Sparks]

try pulling the knob off and this will expose a small nut around the spindle