Cleg Fly (Haematopota pluvialis).
These bite, and I can tell you they hurt!! The flies alight on your body so stealthily that you are unaware of them, until they have sliced into your skin with their scimitar shaped mouth parts and sucked up some blood! Only the females bite, as they need a meal of blood to help develop their eggs. The flies usually rest on foliage and can detect movement. When they have detected you they are very persistent in their search for a meal. They normally feed on large mammals such as cattle and horses, as well as humans.