Looks like Rosebay Willow Herb, one of the 'nasties' in the garden. It spreads by seed and underground rhizomes which you will have to carefully dig up.

Agree. Its a real thug - don't let it flower!

Yes it does look like RB willowherb to me too. Dig it out as soon as you can.

Into battle in the morning then! Thanks for that.

Yes agree can be very invasive if allowed, for me it is a beautiful wild flower I just love it.