Help! St Mark's flies are everywhere

[wakarimasen]

Hi,

I moved to my house a year ago and laid new turf (it's a brand new house). There is some farm land in the surrounding area. Anyway, this year my garden has become a haven for hawthorn or st Mark's flies which seem to be mating like crazy. I can't go outside without getting covered in the blighters. My question is, how long will they be around for? Can I do anything to deter them?

I had never heard of them, but this might help:
www.nhm.ac.uk/nature-online/species-of-the-day/scientific-advances/industry/bibio-marci/