Your post triggered a memory for me - doesn't food for free mention Tansy as being a Fly deterrent?
The following are identified as insect repellents and some specifically fly repellents in my database:
Bog myrtle (the basis of many herbal midge repellents)
Common Fleabane
Elder
Feverfew
Herb Robert
Mugwort
Penny Royal
Purple Loosestrife
Small Fleabane
Tansy
Woodruff