Black fly on Fatsia plant

ants, like nan said. The pot will be full of ants, they are the root cause and should be dealt with. They are past masters at farming black and green fly

Well, whatever they are doing, the black fly disappear off my fatsia.

Kittylester, ants don't eat blackfly, they "farm" them for their sugary secretions, and have been known to move them from plant to plant.
There does seem to be a lot of blackflies around this year, I have several plants affected. You could just spray them with a solution of washing up liquid in water, which gives some control, and is harmless.

We get black flies on our fatsia most years and they get eaten by ants.