This is a nice little perennial Aster which flowers over a long period in late summer and autumn. It used to be known as Aster acris. It has small leaves and purplish blue daisy flowers with a prominent yellow centre. It is a vigorous plant which will spread to form a decent-sized clump. It is probably best to divide it every few years so it doesn't take over. Mine seems to be the cultivar 'Nanus' which grows to only 45cm tall; the taller ones can get to over a metre in height.
The flower stems need to be cut back after flowering to tidy it up. Though most Aster flower best in sun, mine copes with a bit of shade. It suits a cottage garden style, but also fits in with ornamental grasses and Salvia, which are what I grow mine with.
Note that the correct botanical name for this plant is now Galatella sedifolia.