This has just started flowering in sheltered, sunny locations. It is Cymbalaria muralis and my photos don't do justice to the flowers. If you follow the link to the Wikipedia page you'll see what I mean.
It is another unassuming little plant that now grows almost universally on old lime-mortared walls, yet is was only introduced from southern Europe in the early seventeenth century. One nineteenth century writer had it that it was introduced to Oxford from Italy when sculptures were imported and that it rapidly spread around the college walls. One of its names is Oxford weed.
However Mr Grigson has the first record accorded to one of the best known amateur gardeners of his time, Mr William Coys, who was growing it in his Essex garden in 1618.
It has evolved to thrive on walls/rock faces by growing towards light until after flowering when seed is setting; it then becomes negatively phototropic, turning back in towards the wall and any crevices where seed can best germinate.