Wood sorrel is a lovely flower of early Spring, it's an indicator of ancient woodland. Ancient woodland is defined as land that has been continuously wooded since 1600 or earlier (in England. Wales and Northern Ireland) or since at least 1750 in Scotland.
These plants are growing on a mossy wall in Colinton Dell, along the Water of Leith which is an area of ancient woodland. This area isn't entirely natural ancient woodland as some of the trees (particularly the hornbeams) were planted for use in the construction of the mills that used to line these riverbanks and other trees were planted as ornamentals by the country estates that also used to own much of these lands.