RESOLVED.
When I tried dragging down the height of the wall it took down the height of the ceiling affecting other walls.
But I did 'solve' it.
note the existing ceiling height.
Define a ceiling pane only over the wall you wish to lower.
Set the slope to 1/16 and lock it. (That was the lowest i was permitted) Lock the slope and enter the desired absolute height.
So far you get a hole in the original ceiling, a lowered wall with an ugly section of ceiling above it.
First fix the hole:
Copy your ceiling pane and paste it in place. Choose one of them and set the ceiling height to that of the original. That plugs the hole.
In a floor view, select the ugly ceiling pane over your lowered partition. Choose material and select insulation- air gap.
The smaller the section the better as the ceiling does not perfectly get patched because of the slight slope.