Tried an inexpensive way to get rid of those swarms of tiny black drain flies. Mix well half a cup of salt, half a cup of baking soda and one full cup of vinegar last thing at night. Pour the foaming mixture down the kitchen plughole and leave to soak overnight. This should destroy their breeding grounds, food source (scraps, grease etc). Next morning pour a kettle of hot water down. Works a treat so far.