You threw something in there they like to eat. It's really not a problem. The flies are simply doing their job - decompose. Try mixing it around and through a few shovels of garden soil on top, and move the bin away from the house where you can't see it.

they are probably a type of fruit fly and they are part of the rotting process. stir the materials around in the bin will help speed up the process. also leave the lid off and the birds will have a feast.

Whilst I agree they're a part of the composting process I also know they can be off-putting. Try putting corn flour into a fine sieve & giving the surface of the compost a good dusting. It'll collect on the flies, get moist & clog them up

then that will kill them and alter how the compost decomposes. Not exactly helping the compost break down. better to turn the compost and leave the lid off or put a thin layer of soil on the top as Bathgate suggests.

On the other hand if a person really doesn't like the flies, which are probably eating mould so aren't main decomposers, & gives up on composting then the outcome is worse?
Plus with corn flour you get free micro-tempura😄

its the fly maggots that do the main decomposing not the adults. there waste products then provide nutrients for the decomposing fungi etc etc. it is a cyclic nature.

just leave the lid up then when you add extra material to the compost bin they wont fly up in a cloud. :o)

I'm for adding a thinnish layer of soil or lawn mowings, just enough to completely cover the surface - that stops them. Works for me every time.

Hya some good advice there that I will try a few of these and get rid of the problem, Thanks to everyone for taking time to answer, I always use this site as I always get really good advice so Thanks again