Ah, yes. What I needed is combination of mine and yours, i.e.
$_SESSION['category_tree']->build_category_string('ul', 'li', '', $_GET['cPath']{0}, 1, false, true)
The $_GET['cPath']{0} returns the Top cPath only depending on which one was selected and your $include_root=false, $strict=false appears to work. ie. Allows a specific level of the subcat structure to appear constantly, from there another context driven menu appears elsewhere.
Thanks again.
DanP