If you want to try a natural repellent along with the Crovect, Barrier Biotech do a 'Blowfly Repel' spray specifically for sheep. I've got a bottle here, but I'm afraid I haven't tried it yet myself, so can't vouch for it - however I use their disinfectant solution a lot in my chicken coop, and they seem to have won an award for their horse fly repellent, so at least I thought it was worth a try.