Yes, my black sheep get it on their ears and around their eyes and on the top of their heads. It's a real pain in the butt. I've yet to find a decent solution other than re-applying crovect which is annoying. Also you need really to apply the crovect before the flies start biting, I believe once they have made a start it's harder to deter them.
Not all my sheep suffer though, it seems to be the same few each year. We too are surrounded by trees, mixed species, some deciduous some conifer, and my sheep get it from July until October about now.
I've tried spot-on too, which works for head fly on cattle, but it didn't seem to make much difference on the sheep.
I did read somewhere about black sheep also having an allergic reaction to some midges or head fly ... I can't remember the details or where I read that .... hmm ...