If you let it seed (even one stem)it will soon be everywhere, including neighbouring gardens.

It's very pretty but I don't think I'd want it in my garden. It gets big and can spread a lot. It's very common in the hedgerows :)

Pic's a bit dark but its Rosebay Willowherb isn't it? Miles of these down by the river where I live, literally as far as the eye can see... they spread by seed and rhizome, so I'd get 'em out before they set seed, they're not suitable for the average garden, unless that's all you want in it.

Eupatorium purpureum is useful for pollinators, flowers later, gets taller, common name Joe Pye weed.

This would like nice out in a meadow someplace. I'm sure the bees love it, but not in my garden.

The birds love the seeds, but it's way too prolific for anything but huge wild areas.

Thank all, another job to add to my to do list! Have been wanting to rip it out for a while now so that'll keep me busy. Thank you Bamboo for your suggestion of the Joe Pye weed.