Agree with Kerilli, however I dont think that Benzyl Benzoate is licenced anymore. I used to buy it from a chemist (think it is used to treat Scabies in human's). Your best bet would be to buy a Boett rug and keep the horse stabled when the midges are out.
My 1st Pony had sweet itch, i treated it with i think a Carr Day & Martin lotion, seemed to work quite well, allbeit a bit greasy!
A fly rug will help lots too! Unfortunatly, they didn't make such a thing,(well not affordable) when my Max was suffered with it!
You need to buy Killitch, made by Carr Day & Martin, it is basically Benzyl Benzoate. Use your hands to slop it on, fairly liberally. It is not a miracle cure, but it does seem to help a bit. As others have said, if your pony has sweet itch the only rug that will work properly is a Boett. My TB has one and now she can live out. Before that she was a VERY unhappy and uncomfortable horse.
Boett boett boett, if you can afford to buy one it if FAR superior to Benzyl Benzoate. if you go with a different brand of sweetitch rug, please buy a sweet itch rug and not a fly rug - the midges are small enough to get through the holes.
BB is still readily available, just apply daily preferably wearing plastic gloves. If you mix it with baby oil it will help keep it on if it rains. We found Killitch useless but nettex Itch Stop salve ecellent. I would highly recommend a fly sheet though.