Ian, I think your wee campanula is Campanula sartorii or possibly calaminthifolia (but this is frequently sartorii in cultivation). I have had it self seeding in a small area for many years mostly round a now fairly large Juniperus communis compressa and now down between the stones supporting the raised bed. I am always amazed how such a tiny plant can survive in arid conditions and go on to flower. I expect it produces masses of seed and only a tiny proportion will survive to become flowering plants.