Put them on your bird table Shinamae, the blue tits will be delighted.
You must have some flies eggs stuck onto the bin somewhere with something they are feeding on. Try spraying the bin with fly killer, but don't breathe it in, really toxic.
I have had a maggot problem because of leftover dog/cat food, flies had laid eggs on the food. I was being lazy and putting the leftovers into the big bag, without wrapping it up separately, and the eggs had a good feed then hatched into maggots and wriggled out of the bag, urgh!
I've started using the very small swing bin liners in the kitchen now, wrap any food remains tightly in a separate bag, and bin it every day. Shouldn't be as bad now, the flies will be dying off soon.