I use Crovect on my sheep , its the only thing that works well. I've lost a lamb to strike and have had to deal with more than enough cases of it amongst the adults over the years - only last Thursday I had to bring a large lamb in that was struck all round his body and chest and ended up totally shearing him. Poor little b*gger had insult to injury when I had to drag him across the field on his back by his front legs - he wouldn't get up, I was on my own and he was too heavy for me to carry. I did keep stopping to give him a breather (and me!) and to tell him there wasn't far to go - it was across a six acre field

. Thankfully he's looking perky again now.
IME if the sheep have been struck it gets rid of the maggots instantly, and also provides a fairly long-term protection. Try not to breathe it in if you use it though as it's horrible stuff