It seems there’s an inconsistency in the naming convention when going from the properties window to blueprints on this one… just in case anyone else was stuck on this simple issue 
In the properties window, “Intensity” effectively controls the brightness of a light. However, this property is modified by using the “Set brightness” blueprint node. As far as I can see, the only way to get the “brightness” is to get the light’s associated light component, and then get the intensity variable directly (which is marked as readonly, so cannot be set directly).