I did come across a work around:
Turn off one of the conflicting story markers in the story settings.
Use the library part 'ManualStoryMarkerAus 13' for the story level that you just turned off. This library part allows you to stagger etc, exactly what should be doable with the built in story markers.
(With the 'Aus' in the title of the library part, I hope that this is not limited to just Australia, can anyone confirm this?)
Of course you, and anyone else working working on the file, will need to remember that this work around is in place, and that it will need to be adjusted manually if the projects story levels change.
Mike