I figured it out....

The penny has finally dropped with List Styles...thank goodness for that. It has been a battle and many tears have been shed but fingers crossed, its plain sailing from here on in. I was coming at the task from completely the wrong angle. If anyone has the same issues, I resolved it by setting up three List Styles tied to three different sets of Heading styles each with a set of paragraph styles based upon them, so that the paragraphs follow the same numbering system. I've used one 'set' within each document.