Hello to Inkscape forum,
I'm having a problem when I try to edit "any" lines in this respect: the line opens apart from one edge to the other, this new shape (for wont of a better description) can even be filled with a pattern etc. as though it was an object rather than just a line...any thoughts?

look in your toolbar for the word "None" after "shape:". It sounds as if you have it set to something else, most likely "Ellipse"