Skin So Soft is used by the military for the same purpose, unlike DEET it doesn't damage vital components of delicate devices (like cameras, binoculars, computer-guided weaponry etc.).
I've just bought some of the non-DEET stuff from Millets, not yet had the chance to try it out.
Where Clegs are concerned, you can normally spot them as they home in on you and give them a clout (but do it properly or they'll bounce back and come after you again, they are persistent). They bring me up in a huge welt as well, so I've got to know my enemy. I was once hunted by a large one (deer fly) while snorkelling in a river in Canada, every time I surfaced I could feel it land. I won in the end, it became food for a young Salmon when I crash-dived with it still on my head.