Thanks for the reply. The trouble is that I am only using the CAD program (Blender) as a throughput to change file types and do a bit of scaling. That is the limit of my know-how with that program and, being lazy, I am looking for a shortcut to avoid having to learn it properly just for a simple function like this.
I guess I am hoping there will be a simple instruction in the CAM program to just machine the top face, if anyone knows. Presumably this instruction is common to most CAM software?