Have you tried ant powder? At least that wouldn't kill the grass as boiling water does. Many gardens are full of little black ants, often under patio slabs, and they don't usually cause problems unless they nest under a shrub, which can damage the roots. I wonder if the flies you see are actually the ants departing on their mating flight, which they do one day all together in the summer? They are mostly males, who don't return to the nest once the queen is fertilised. You could also try the bait which they carry back to the nest and which kills them inside it. But if you have a lot of ants in your garden it is really a losing battle.

Actual flies don't form nests, but may collect around pieces of dog do, rotting fruit, and the like. I would suspect that they are something other than flies, such as winged ants, as Steragram suggests, or blackflies, or webworm moths, or even hornets or burrowing bees. Unless you can get good close-up pictures, especially of them coming out of their "nests", you will probably have to engage a good exterminator to identify them and engineer a solution. Ants are pretty straightforward, but unfortunately, most of the organic remedies simply move them from one place to another, especially if they are found in a large lawn. One question that you want to ask yourself is what are the critters eating? I would follow a trail leading to the nest back to the food sources, and see if you can eliminate their source of supply. As a last resort, you can do extensive reading of various chemical insecticides, and choose the one that seems the least damaging to the environment. Be forwarned, though--many ant remedies will kill beneficial insects as well as the ants, and you may wind up with more troubles next year, due to the imbalance.

There is now a nematode solution you can water on for ants - google Nemesys and look at the range of things they do. Not sure when's the best time to use it though, usually its more effective at different times of year and certainly before the soil temperature drops in winter. I just checked and its called Nemesys No Ants...

Thanks for the anwers. From your descriptions they definately sound like winged ants rather than bees or anything else. Blocking their food source is a good solution. I have a week off in late August and will tackle it then. I suspect that the neighbours cats droppings may have something to do with this. Thanks again.