Coopers fly reppellant is really good it will last your horse practically all summer, kills flies and leaves the horses coat shiny. Its a green/yellow box and about £25. It really works!

My horse gets really bad reactions from bites, that swell up to sometimes bigger than the palm of your hand.  I have tried all sorts of flysprays (brand and homemade) and garlic but nothing seems to work (and my horse hates having a fly rug on).  I was going through loads of fly spray, but then my friend suggested using flyfree by global herbs supplement.  It takes about 2-4 weeks to get into the system but i can't believe the results. I have not put a drop of flyspray on her in 2 weeks, and she is totally bite free :D  It is well worth a try here is a link http://www.globalherbs.co.uk/equine/flyfree.html at £12 a month it aint expensive either. hope it helps

i know someone whos horse got bitten so bad that the bites would swell up like a baloon! The only thing that worked to really keep the flys away was vinegar. I know the smell would put most people off but it really does work! hope this helps