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 ...

I have a similar issue with a two level project. In my case things are happening differently depending on the level selected on the tabs on the grid plan.
With level 0 selected everything (grid plan view and 3d view) displays as it should. However...
With level 1 selected and 3d view > display ...