Jersey Tiger on white Phlox (probably some variety of P. paniculata?). Not a butterfly but a moth that flies in the daytime as well as at night. The Jersey Tiger is thought to be have migrated from the continent to our southern shores.

 Considered a rarity in Victorian times it is now well established in those coastal counties and said to be expanding its range rapidly in recent years. That seems to be the case in Hertfordshire. I saw one for the first time a year or two back; this year I am seeing them regularly.

 The black and cream stripes on the forewing are striking. The underwings are visible in flight: vivid red or orange with black spots. At first glance the blur could be taken for a Comma or Tortoiseshell butterfly.