Thanks, I really appreciate the rapid response.
We have looked at the Window by Ratio - zero option. We went in a slightly different direction that appeared to give us explicit control over the definitions of the materials that we wanted to assign.
My surprise was that after converting the faces into geometry via the Viz Quick option we had geometry that created a closed volume - just walls, roofs, floor, no apertures / doors. But, when we took that apparently plane geometry and made it into faces, it retained the ‘ghost’ of the apertures. Frustrating in that we could not understand what we were missing in the logic of the script.