I've always used Vetrazine.
Keeps for a while too. Once you get to know your ground you will know how bad it is for flies. I have a fair bit of grazing by some lakes so I tend to treat every 10wks in the season, even on Wiltshires. I've barely ever seen flystrike, but I don't know if thats to do with my prevention routine or the fact that Wilts dont get struck very easily.