I think I must have had a few vertex selected off the top level when I flattened the bottom layer with S,Z,0.
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You probably had those faces selected and pressed F in error. That is what makes that happen when you want it to!
If you go to vert mode, you can select pairs that should be connected and press J for join. I would do all in one direction first then the other. That should replace the individual faces.
It will not have been S,Z,0. Or the verts would be there but flat to the Z axis.
Thank you for your advice, it worked perfectly.
I learnt another way to use that useful tool (J).
Chears
Pete
