It's a campanula but sorry not sure which one it is, because of where it's growing could be poscharskyana, but I'm sure someone will be more definite for you

You can buy plants at the garden centre - sometimes labelled Campanula muralis, sometimes C. porscharskyana. Don't get mixed up and buy C. carpatica by mistake - throws out long flowering shoots and a very untidy grower.

This is a campanula portenschlagiana. For seeds just google "campanula portenschlagiana seeds"

Very helpful, have ordered some from Ebay.
Thank you very much

My deepest apologies - I got the names mixed up, Swissy is absolutely right - campanula portenschlagiana is synonymous with campanula muralis, not porscharskyana.