Can’t remove wall mount for wired cam

I want to move my wired security camera but I can’t work out how to remove the mount from its current fixing. I’ve twisted and easily removed the actual camera. The fixing screw on the mount appears to be missing but it is still stuck fast to the base which is screwed to the wall. I’ve watched the video and read the manual on how to install it, and it appears that it just clicks on, so I assume it should pull off? But no amount of twisting, pulling or prising seems to help. I can’t remove the mount to unscrew the base plate. Any ideas?