At some ,
Phil Layton typed:
trying not to sound patronising You don't mean the plague of
Hoverflies do you?

Probably cupra. Whose responsible for them ?

Phil

Not sure - there was talk of a European invasion in the South East, but there's loads here in Northants so I doubt they would bother to fly all the way over here !

My guess is the predator/prey cycle - plenty of greenfly for the larvae to eat!