i transplant any i get after they have flowered.

Anytime. Dig them up and stick them in their new home; we do this regularly, even in full flower. They are thugs and will tollerate any treatment.

Oh dear. I can't even persuade them to grow in the first place!

I have that problem too Merlinbd. First time a clump has come through for the 2nd year. I am keeping my fingers crossed for the future.