This wall has large clumps of  the small yellow trumpet-like flowers of Yellow corydalis  (Pseudofumaria lutea) growing along part of its length.  Originally the plant came from the foothills of the Alps but has been recorded growing wild in Britain since 1796 and likes rocky places with very little soil such as walls or paths where its tiny roots and seeds can infiltrate gaps and often ants carry the seeds to their nests in a wall.  It brightens walls from spring until autumn.