This is Campanula (Canterbury Bells). It comes in blues as well and self seeds rapidly. It's a very pretty plant and I don't mind it in my garden at all.

There are very many Campanula, these are clearly one of the numerous tall varieties but not sure which one.

many thanks for your replies.

I agree Campanula (Canterbury Bells) I have blue ones

I reckon its Campanula alliariifolia

I dont think it is Bamboo? the bells are not long enough and they are too wide?

that's really weird - I just googled it and the flowers look different from the pic in my books here - my book shows flowers that look exactly like this, but on Google, they don't. No idea which one it is then.

I have grown blue ones like this since 1978 and always known the as CB