Thank you...that's helpful. I will try restricting the times they are in the field, and perhaps get it mown. It sounds as if if I'm patient seasonal change will sort the problem, and I think I'm probably just about out of fly-strike season now it's beginning to get cooler...or shouldn't I bank on it?
I Treated them with ivermectin so I suppose fluke is a possibility, though they do get better if I keep them off the field.