Actually I can now answer my own question.

After I posted my original question here I went browsing for a couple of
hours. A vaguely related issue on opening workbooks gave me a hint.

The problem is not in the code at all. It lies with the definition of the
key to which the macro is attached : using a shifted value causes exactly the
behaviour I observed. I was using ctrl-shft-q and changing this to ctrl-q
fixed the problem. Simple when you know it.

This is apparently a bug in Excel.


Cheers,

Dave