So I decided to experiment a bit with the top with the interface. Rather than subdivide I tried other options.
First I looked up / remembered that the left mouse button puts that target places, and that is important. So I selected the geometry faces on the roof in edit mode then pushed Shift + S to move that cursor to the centre of that selection.
Then instead of remove duplicates, I tried merging. That created a point that I then translated up the z to create a roof.
My target engine of choice is Unreal - notes on their wiki here about exporting for Unreal:
https://wiki.unrealengine.com/Static_Mesh_from_Blender#Exporting
I’m short on time so want to press on with learning more blender rather than switch context back to Unreal. Sooner or later though I’ll post progress of that.