I think I may have cracked it, but then again, don't get your hopes up. Following a link from a page on balsam, as submitted by Pipkin I found Mirabilis Japala [or was it Japa something else], otherwise known as a 4'clock plant. One of them looks exactly like yours and it habits seem to match. Alas it isn't the same as mine so I am still none the wiser. I think it is invasive in some climates, but not in ours.