Ok, there is an add button, but no joy. When I right clicked, there was an add option, but the atlas name and the add option were different, see attached image. Also clicking on add didn't do anything, just brought up the UIAtlas/Sprite details in the inspector. Note, the add has the right name, the atlas name on the top is the wrong one, but the list of sprites matches that atlas name, not the one I want to edit