Name of this purple feather duster rail track side flower?

[NewspaperTaxis]

One of these has emerged in my garden after a clear out of brambles - it was always there really but overshadowed.
This plant with its purple feather duster flower is seen in droves alongside the railway tracks as you head south from London into Surrey, all through Balham, Raynes Park etc so I imagine the train movement picks up the seeds and dusts them alongside the tracks, encouraging them to grow.
But what is its name? It's a sort of weed but a rather attractive one.

Strange thing about the buddlia. It smells lovely in the garden, but cut some and take it indoors - and it's like you've strayed into a cats home!!

Yes loopy they are never sold by florists perhaps that's why! I don't pick the flowers because the insects need them. They die when pollinated!

I have a buddleia in a pot on my terrace specifically to attract bees and butterflies!

My buddleias have also never self seeded or spread and fine to have close to buildings, it’s not knotweed. They can grow very tall but a pair of pruners will soon deal with that.

I really need to get one of those telescopic pole pruners.