Mmmmmmmmm... under the help menu, "Merging Duplicate Individuals" I found this:

"After completing the merge process, you can always undo the entire process and restore your file to the way it was before you started merging as long as you have not exited the program yet. Once you exit the program after merging, the changes become permanent; the only way to restore your file to its original state is to restore the backup file. "

Family Tree Maker version 11

Of course I had already exited the file, so therefore it looks like my only course of action is to delete and re-enter...... but hope this mistake of mine helps someone else.

So, guess the lesson here is, as last resort, to always read the instruction manual.!!!!!