I am sure you use fly repellant already but I found Endure, it smells good, it has lanolin in it so does not cause dry skin and it really works. I noticed you're from England, correct, so you probably don't have the same stuff we have in US. I spray her every morning before work put her mask on when its time for bed I spray her again take her mask off. I make sure to spray some on my hands and rub on her face and nose.
Good luck and I sure hope she's feeling better soon.