Are you sure it is greenfly? The most usual pests on brassicas are slugs, caterpillars and pigeons so its difficult to suggest a remedy without knowing which it is. Caterpiullars are easy to identify as you can see them on the leaves. Pick off what you can see and net the plants to prevent the butterflies laying eggs. slugs you can catch at night if yougo out with a torch or you can use pellets. Pigeons are a pain but netting would help.

enviromesh.

Thanks scrumpygral