Green veined white butterfly, smallest of the whites and the one that I at least need to see the underwing for a definite ID. This one was systematically going from flower to flower on a patch of blue alkanet, staying long enough on each flower for me to get a shot in.
Eventually, it did give me a glimpse of its underwing. The first picture in this set, but actually the last one I took before someone came along the canal footpath with a dog, and the insect retreated.