Ok. Apologies to Jungle & Mathias.
Error is in the user not the program.
I was trying to rename using the 'Search and Replace' inputs on the right hand side, not the 'Filename' input on the left hand side.
Once I realized that I could just type in the new name and add rename rules it worked brilliantly.
Oops!
In my defence, the layout is a 'little obtuse'. Or maybe I am trying to justify my error.
Not sure how I would change it. Perhaps Mathias you went through the same questioning when you designed it? The end result may in fact be just what is needed.
So the end result is: the function is perfect and like everything else about MultiCommander is to be applauded.
Thank you. Well done.