Thanks to Tiggrx - looks like an indigo species, Indigofera, possibly I. amblyantha.

I'm fairly sure this is not a British native but it grew (presumably from seed?) in someone's garden a decade or two back. Since then it had been grown from seed and passed around the family .... does anyone have any idea what it is. This bush is about 2m high - can grow higher. Doesn't seem to bother about soil type.