OK I get it now, it works as you say providing the entire MergeArea is
passed to the function (which means of course you need to know in advance
it's full meargearea) or pass say activecell.mergearea. Previously I had
only been passing a single cell and so it didn't do as expected.

I had a version that used a single cell, but that would be inefficient, as
I already know the merge area.