It's Koelreuteria paniculata - a reasonably fast, small to medium
sized deciduous tree native to N. China and has been grown here since
the mid 18th.C. It is pretty hardy although the young shoots are
prone to damage in spring, so a sunny sheltered spot is best. The
short lived flowers are followed by inflated, translucent seed pods,
which are almost as attractive as the flowers. The foliage turns
clear yellow in autumn making this a good 'all-rounder'. It is not
overly fussy about soil types, but as with most plants, a humus rich,
moist yet well-drained soil gives best results.