Not sure,but do you think it could be Lunaria,or as it is commonly known as Honesty?

i think its honesty as well.yes it will self seed prolifically, my mum has it in her garden, mainly because she likes the silver penny seed heads but its not a huge problem,they like shade and are easy to pull up if you get to many.

Yep,Lunaria. I like it. It grows in the really dry shade under my trees and I love the purple haze it makes and then tops it all off with the wonderful silver seedpods. If it's a woodland garden you have, then I'd say this is a good choise.

i have a few dotted about the garden. good colour in deep shade too.

Thanks everyone, as usual, you've come up with the goods