@LittleLionMansMummy,
I've known about horsefly bites for years. The reaction is very different from a sting due to how the bastards do bite as opposed to puncturing the skin. I had a bad reaction in 2013 to a bite on my arm that turned into cellulitus and I was quite poorly. In the end I had to be admitted to hospital and IV drip.
So, am on anti-biotics now because of my history.
It's so important particularly for children whom may be bitten as the itch can puncture the blister.
Horsefly bites should only be treated with warm water and/or antihistamines for home treatments. First sign of spreading, should be antibiotics. The blister should not be popped.
I went to the pharmacist whom confirmed the bite was horsefly. Then he told me to call GP and because of my history, straight on antibiotics.
Horrible fuckers.