Mystery plant

Does anyone know what this plant is that has either come under the fence from next door or broken through a gap? It looked quite pretty last year, but this year it's looking more alarming with those huge tall stems that have sprung up just recently!

 

Wow, thanks for the super fast response Marley, and for the identification. As you say it self seeds I won't feel too guilty about cutting it right back when the pretty flowers have finished. It's already quite big - don't want too many more of those in my small garden!