Along the edge of a field of wheat there were lots of tiny flying insects seemingly attracted to the buttercups lots of them mating. They were very small less than a cm long and very slender.
Using the trusted insects of Britain guide and then searching on google for photographs they are Cepheus pygmaeus, common name European stem sawfly.
The larvae of this sawfly are stem borers and can do damage to crops. There are 12 species of the Cephidae in Britain.