The pinkish fluff on the fly's abdomen is the fungus
Caution: Do NOT use iSpot to identify fungi to eat!
Some fungi are very poisonous so a mistaken ID could have serious consequences.
No interactions present.
These fungi can have amazing effects on the behaviour of their host insects, causing them to move to the tips of plants before killing them, so that the fungal spores are more readily dispersed.
I've been told in the past that you can't be 100% certain that a fungus on a fly is the species Entomophthora muscae, apparently there are other fungi species that can infect flies in a similar way.
Entomologist and biological recorder
Lat/Lng: 52.9, 0.9
OS grid ref: TF9842