We have been plagued by them this year and I have been bitten twice by them. They are horrible things.
Thankfully they are not as quick as houseflies, so you do have the opportunity to kill them when they pitch on you, although this is not going to save you being bitten if you are riding at the time.
I did a bit of googling after being bitten and one person said rubbing your own saliva into the bite can help relieve the swelling as it contains anti-bacterials and another mentioned rubbing bracken on the bite.
I was hoping that the recent unsettled weather would have washed them away, but I was buzzed by a couple whilst out washing the car yesterday, sigh!