Looks to me as Verbena bonariensis or a similar plant from the verbena family. I only know the Dutch name ijzerhard for sure. Searched on that on Wikipedia and changed language.
Also have it in the front garden. Bee's, hoverflies and butterflies love them.
Ha, yeah that's it! Thank you both!
Looks like a buddleia, but it would be useful to see leaves to be sure
It does not have many leaves really, but they look like this:
edit: But some kind of buddleia seems right i guess, thank you.
Yep, classic buddleia, grows anywhere, obstinate and difficult to get rid of 😁
Purpletop vervain
https://en.m.wikipedia.org/wiki/Verbena_bonariensis