Eupatorium sordidum or Bartlettina sordida
or Ageratum corymbosum
It seems to have many names, some of which I have used in the title. And it appears there are quite a few varieties though I haven't seen any in the plant nurseries.
I am going to try and propagate this one using tip cuttings - even though a small tip is hard to find on a plant with such large leaves and branches.