OK. I managed to salvage my project. I saved my building as an EBE structure. Then I opened a fresh empty level and dropped the structure in. From there it did allow me to edit it so I made the edits I wanted and again saved as EBE.
Finally, I opened my project and deleted the problematic EBE house. Then I put the newly saved EBE structure in the scene and dragged it into place. I need to reposition some of the windows, doors and furnishings but... phew, I think I saved the project.
I still think there is a bug behind all this.
Julian - increasingly disillusioned and jaded