I found this procumbent yellow-sorrel (Oxalis corniculata) growing in a corner of Battle market square. It is the form with dark leaves known as var. atropurpurea.
It is not a native plant and, although found all over the temperate and tropical world, no one seems to know its original home. Its success is partly due to the fact that its explosive seed pods can expel the seeds for up to 2 metres.
Like many other Oxalis species the leaves are said to be edible and can be made into a herbal tea.
Often regarded as a pernicious weed, procumbent yellow-sorrel is said, by the Royal Horticultural Society to be "strongly resistant to the range of selective lawn weedkillers." Hooray!