Defo sound like it could be a dead mouse.. But you can usually smell dead mice... Funny time of year for flies... But Blue Bottle Flies breed often during the warm months. Both larvae and pupae can live through the winter, but adults die when it gets too cold. Blue Bottle Flies sometimes lay eggs in the wound of a living animal. The larvae, when they hatch, eat from the host animal. This is called "myiasis" and can cause infections in the animal. Source WARNING Nasty pics of Blue Bottles Could be caused by heating being turned up due to the cold snap.. Fox.