It seems to be hit and miss Simon, i've tried SSS through to Deet and nothing keeps em off me. 
I also react very badly, each bite takes on the form of a gobstopper beneath my skin and lasts for weeks.
I'm a gardener so used to creepie crawlies having a nibble at me, but nothing comes close to those winged bastards for causing misery, i wish we could nuke the chunts, obliterate and exterminate them, or at least stop them feeding on me.
Include a little history in your walks. Pecsaetan - Ancient Derbyshire, Staffordshire and South Yorkshire - http://pecsaetan.weebly.com/