Aesculus x carnea, Red Horse Chestnut
A hybrid between the European Horse Chestnut Aesculus hippocastanum and a small American tree, the Red Buck-Eye Aesculus pavia. The hybrid isn't very well-formed, the flowers vary a lot in colour and development and the leaves are always a bit warped and sticky with aphids. But they do make a nice Fleurs du Mal study, the pink-striped anthers breaking to chocolate. Some flowers (though none in these pictures) have long white snaky stigmas too. Shaded flowers often lack any red colouring.
Red Horse Chestnut (as Alan Mitchell vengefully noted) "suffers from a canker, so is fortunately not long-lived". But since he wrote those words, bleeding canker has infected most of the European Horse Chestnut trees in the UK.