Perennial, speading by seed. Looks similar to other smaller willowherbs
Weedkillers to use:-
Dichlobenil prevents emergence of seedlings among established woody plants, for up to one year.
Paraquat, Diquat kills top growth on contact, use at seedling stage.
Glyphosate, systemic action killing the whole plant, do not use when in flower as the seed may be produced before the plant is killed.

Follow these links for further details on Weeds, Weed Removal and Weed Prevention.