Thanks Tig, just needed a little lateral thinking.
The workflow I was using was to create the walls with dline.rb, which obviously doesn't create the floor slab, I was then telling myself I needed to trace the room boundary. Using what you have suggested it is easy enough to open the wall group, redraw one of the inner edges of the wall to create the face that forms the floor. Select the face WITHOUT the boundary edges and edit>cut. If you select the face AND boundaries the edit>cut will destroy the original geometry. Close the wall group and then paste in place to create the floor which can then be pushpulled, grouped & layered as required.