Given the amount of attention they've been affording me I wonder whether I should, perhaps, start neighing.
Their modus operandi lacks subtlety. They land on either skin or clothing. They prefer skin but will put up with clothing if ready access to flesh isn't available.
They then rip a hole in the skin. It's not a sophisticated manouvre and it does hurt. The idea behind the pain is that the victim focuses on the injury rather than the cause of the injury, this leaving the horsefly to leave the scene unharmed.
Once your flesh is exposed the horsefly then has ready access to your blood and will return, at will, to feed.
Apparently they are repelled by insect repellent, but I'm not convinced.