Hi Geordievik, they sound as though they could be fungus gnats if they are very tiny black flies. They like damp compost & lay eggs in your plant pots which develop into tiny little creamy/translucent grubs which eat the plants roots. They're not such a problem if the plant is well established but will kill off any seedlings or plants with not much root to start with. I'd repot your plants washing the old compost off the roots & using new compost & try to keep the compost on the dry side so it doesn't attract the flies. They probably didn't come in the compost unless it had been stored somewhere very damp & the bag had lots of airholes in it, but they seem to find thier way into the house by all sorts of methods.

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