Many thanks Hans for pointing me in the right direction.
In spite of your help I was still having problems with this (trying to find ceiling and floor properties etc) until I realised I hadn't actually created the rooms on either of the levels. I had only created the walls and had assumed that by doing that they were rooms. My confusion arises from a different home design program I'd been using where creating the walls and sealing an area automatically created the room.
All is now behaving as intended.