What is this Plant?

Kitty52

I had this 'plant' in a plant pot in the cold frame over the winter.  It was growing well and I put it in the garden where it has also thrived.  Unfortunately I have no idea what it is and hopefully its not a weed!!  Does anyone know what this is?

josusa47

Looks suspiciously like rosebay willowherb, but I could be wrong.  I think it's a wait and see plant.  If you don't like the look of it when it flowers, you can get rid before it goes to seed.