The Gardener's Almanac

Our pot plants have a lot of little flying midge-like insects on the surface of the compost and the roots being attacked by

small maggots with black heads.

How can we get rid of them?

The small midge like insects are sciarid flies, and the maggots eating your plant roots are their larvae.

The insects can be killed by lightly spraying the compost surface with a suitable insecticide, and the maggots will need a form of

biological control, e.g. nematodes.