Hi Kate, yes it is probably greenfly, because aphids are common just now and I have seen many less ladybirds than usual, to eat the aphids, could do with some!
Aphids are to some extent a sign of plants under stress from eg dry soil, wrong temperatures, weak variety or lack of ventilation. I have some in different parts of greenhouse and tunnel, for all those reasons! and am not too worried as they usually diminish as summer goes on, with predators arriving.
However if you have a lot and are concerned, you can spray soapy water, I have never done it and perhpas someone else can suggest a soap or other additive?