Within the current legend properties you are only able to enable the option 'Only show classes that are visible in the current map extent'. This works brilliantly until mystery gaps appear when classes aren't visible between pages.
So the idea that I am putting forward is:
Rather than removing the classes when not visible, it would be good if there was a way that they could be 'faded out' thus maintaining the structure of the legend whilst illustrating to the map user that there is not any of that class visible in the map extent?
I know there may a be a work around, but I can imagine it being very long and complex.