As the others have described these little flies are all part of the "composting" cycle.... I just lift the lid off my bin, put it down on the ground, go away for a couple of minutes which allows the flies to settle down and not be so angry at being disturbed, then put my veggie waste in (veg peelings, raw fruit waste, toilet roll/kitchen towel cardboard tubes, scrap bits of paper, plant trimmings etc but definitely nothing cooked). The lid goes back on and a few months later I have the most wonderful compost for my garden. I actually have two compost bins - one is a plastic covered polystyrene *cube* which gets very warm and produces the most compost in the quickest time and the other is one of the council sponsored *dalek shaped* ones, just a big plastic tub really. Although both of mine are on elevated racks (not soil) the worms have managed to find their way in and are busily munching away inside - they seem to love the cardboard tubes from inside rolls of toilet paper ......