It's jacobs ladder - polemonium. It likes sun or part shade.

looks as if it will be fine there and self seed

That is very pretty. It comes in white and pink too.

I agree, it is Jacob's Ladder and it self seeds so you will find it coming up everywhere! I like it but some people think it is a bit of a bully:)

its a nice dark form and it may well be the variety called 'Purple rain'. it is a good doer as they say.

Thank you. I'll just leave it where it is then :-)

I have this in my garden and the bumble bees love it! Remove the spent blooms and it should keep flowering until August.

Thanks, Peonyrose. I haven't noticed any bees on it but that could be because it's growing near a euphorbia which is always covered in them. I shall remember to remove the spent blooms though :-)