One thing that may help but I found odd; for years, our neighbours had a garden full of mares' tail, but our garden remained clear. Then they took down the privit hedge and replaced with a big fence and by next summer it had spread well and truely accross the perimeter and we never erradicated it (but mum didn't like using chemicals). I'm not suggesting you plant the whole thing with privit, but maybe once you've got it under control, if its coming from somewhere, this may stop it comming back..?