Horseweed ....

... posh name Conyza (previously Eringeron) canadensis.  As its name implies, it originates in the Americas, but is widely naturalized in Europe.

It has appeared at the front of our grass bed - and has taken a little while to identify.  It is an annual, so this particular plant will die once it has flowered and seeded.  But if all these flowers are allowed to form seeds and spread, that could result in a massive invasion of further plants, so maybe I need to pull it out soon.

But first, I thought it needed its moment of fame as an uninvited garden guest, and for Wildwed (thanks Cailleach)