But which Erigeron in particular? It is a fairly big genus with everything from impossible to grow alpines to fairly easy border plants.
E. karavinskianus is the self seeding thug one which is all over Great Dixter.

Spanish daisies are the E. karavinskianus so finding them under their botanical name will be easier. they like dryish conditions, crocks in paving base of steps etc self seed freely if they like your garden, fully hardy and generally trouble free. having said all that they will not establish in my garden :o(

Hello Owdboggy,

I am after the one you mention I think. Done more research and it is also known as Fleabane. You see it on all the craggy walls at National Trust Gardens and it looks lovely so yes, I need it to self-seed and be all over! Thank-you.

Hello Seaburngirl,

Thank-you for getting back to me. I note all you say and will look out for some already in pots, failing that, will have to sow from seed this spring.

We bought one at Grosvenor Garden Centre last week in a 5 litre pot for the grand sum of 99p. (selling things off, they were).