Small flowers for top of stone wall
 

You should be able to get the Campanula seed at Middletons, see link below:-

http://www.garden.ie/supplier_details.aspx?id=156


I also endorse Erigeron karvinskianus 'Profusion' along with others
here. It seems to enchant everyone who sees it.
Look out for the smaller wallflowers (Erysimum spp.), which are good for
insects and are real wall-growing plants!.
Try Thrift (Armeria spp. and cvs.) which will cope well and flower for
longer when dead-headed.
I love Veronica 'Crater Lake Blue'. It flowers late spring into summer
with the most stunning blue flowers.
You may even like to try Primroses (Primula vulgaris). Although they're
essentially woodland plants, I find they seed themselves in all kinds of
cracks in my garden, both on walls and at the base of walls. They're
often very early flowering and therefore good for bees and other insects.

--
Spider
from high ground in SE London
gardening on clay