This is a patch of Slender Speedwell (Veronica filiformis), which I photographed by the footpath alongside the River Frome near Frenchay this afternoon.
Like its more common relative, Common Field Speedwell (Veronica persica), Slender Speedwell is not native to Britain. Originally from Turkey, it was introduced into Britain in 1808 as a rock garden plant, although was not widely grown until the 20th Century. It has since become widespread, with the only significant gap in its British distribution being in the highlands of Scotland. In Britain, it is often found as a lawn plant, and tends to spread from mown cuttings, rather than seed.
Like Common Field Speedwell, Slender Speedwell has relatively large (for a speedwell) pale blue flowers, each with a whitish lower petal, and these are arranged singly on the end of long thin stalks (pedicels) arising from the leaf-axils (the point where the leaf-stalk, or petiole, joins the main stem). The pedicels of Slender Speedwell are much longer than those of Common Field: at least twice the combined length of a leaf and its petiole. Also, the leaves are round to kidney-shaped, compared with Common Field’s more oval or pear-shaped leaves. The mat-forming habit and bright green leaves are also good clues to its identity.
For more images of Slender speedwell (and all the other British speedwell species), take a look at the links on the Web Resources page.