Delonghi Desk Fan - tear down assistance

I have one of these that is about ten years old and there has been a build up of friction from dust or perhaps a lack of lube. Was breaking this down for a clean and lube / oil but cant seem to figure out how to remove the swing control knob. It is the last piece to the puzzle for removing the motor and bearing housing. I have tried to unscrew it both anti-clockwise and clockwise (which only seems to unlock / lock it to the swing motor shaft. as you would expect), but it seems to be held in by something else. There are no screws or bolts visible and I have had a feel around for a circlip but doesn't seem to be one (and no room for one really). Has anyone got experience with these fans and have an idea how to remove? thanks