Sacha wrote:
Maybe the
'wild' ones seed more freely than the posh ones.

Indeed. It is well known that cultivated buddleias will attract less
butterflies than the wild buddleias which incidentally self seed. The
cultivated buddleias do not self seed so easily and the paller ones do
attract butterflies more than the dark varieties. Mine, and I've had
plants for years, have never self seeded. My garden is surrounded with
a dry wall and only valerian, wall flowers, sempervivens and lobellias
flourish.