I can grow purple loosestrife! Sloping site, catches the wind, sun most of the day on 2/3 of the garden, but not as far north as you. Right up on the Pennines though so we always get the bad weather. Perhaps I just started out with some poor specimens. Will try again this year. I have seen some growing at the side of the road up the hill from me. Perhaps I will go and pinch some!