Try Clik -I find it much better than Crovect. My sheep are in fields surrounded by trees & there are always flies buzzing around.
I only have 14 sheep & swear by Clik - it is slightly more expensive than Crovect, but lasts for 16 weeks, so it actually works out cheaper in the long run. Although I see blowfly eggs on the sheep, they never hatch & having used Clik post shearing for the last 2 years, I have never had fly strike on any on my sheep post shearing. Because of the action of Clik it protects the whole body by binding with the grease in the wool & skin, so it works differently to other products on the market. See
http://www.farmanimalhealth.co.uk/sheep-clikHowever, I did use Crovect prior to shearing this year as it only last 6-8 weeks, but found fly strike in 4 of my sheep, 3-4 weeks after treatment. One lamb was struck underneath in its udder area & I discovered it when we turned it to check its feet, so its not just the upper areas of the sheep prone to fly strike. Next year I will use Clikzin for the 6-8 weeks prior to shearing as I need something more effective & I feel 100% confident that my sheep are protected. Even if caught early, like mine were, fly strike is nasty & very unpleasant for the sheep.
I bought a 0.8l bottle of Clik last year with a long expiry date & having stored it properly, I have used the rest of the bottle this year & it appears to have worked well - eggs seen, but not hatched!