An orange-red bulge on the edge of a hawthorn leaf betrayed the presence of Dysaphis crataegi. This 'species' of aphid is in fact a complex group of closely related species requiring identification by an expert. I am content to leave it as D. crataegi. Part of its life is spent on hawthorn (the primary host) and it then moves to a secondary host, which can be any one of several members of the carrot family such as Cow Parsley, abundant in the pocket park.
The aphids are concealed, safe from birds, in this leaf roll.
Stefen Hill Pocket Park,Daventry. 25 April, 2019