In reply to Robert Durran:
I'm sure they've been around for a long time, judging from old threads on UKH. I haven't encountered them in such numbers until recently, but perhaps that's also because I haven't been to the 'right' hills. From talking to people on the hill, I've found we may confuse them with flying ants, which are also around at the moment. The flying ants seem to hang out at summit cairns, whereas the keds are much more widespread.
I first became aware of them 15 years ago in the Pyrenees. The locals called them cow flies, but we called them robo-flies. As Williecleuch points out, they're virtually indestructible. If you swat them like you would a normal fly, they just lap it up. If you brush them off, they're back on you in a millisecond. My technique is to clap a hand over them and pull it back until I can squash them thoroughly between finger and thumb.
Cheers
Al