When I was a kid I found this growing along the Wye in Herefordshire (http://apps.rhs.org.uk/advicesearch/Profile.aspx?pid=480), and got really frustrated as being an introduction it wasn't in my wild flower book. It's only since I got the allotment that I've realised what a dreadful weed it is; it spreads like mad in damp ground.
The one good thing is that the seeds are short-lived. It either comes up the following spring or not at all. I've been trying to exterminate up on the railway embankment above the site. There's definitely less than last year, but still far too much. A good frost would have exterminated it once it was up, but we didn't get one, and it doesn't look too likely at the moment. A dry summer could do it, as the embankment is far from being a good habitat for it. Otherwise, I'll be grubbing about again pulling out every one I can find, and this year I'll bag them so they can't set seed as they lie dying.