What do you know about the Dobunni iron age tribe of Britain?

The Dobunni were a British tribe who lived in the west midlands of England (although it wasn't England then!) and they produced coins between about 50BCE and the Roman Invasion in 53CE. Where did the Dobunni come from? Were they Celts or indigenous Britons. Were they descendants of the builders of Stonehenge and the Uffington White Horse or were they invaders from Gaul or Iberia?