I have looked on a map and judge that I am between 400 and 500 metres away as the fly flies.
The next question was - how far can they fly?
How Far Can House Flies Fly? | University of Kentucky Entomology
“How far can house flies fly?” becomes an important question. There is no absolute answer but house flies can easily move one to two miles
If large numbers of flies develop at a site, competition among them for larval breeding sites will force a portion of them to move. The need for effective fly management at major or potential breeding sites is obvious. This can be accomplished in large part by sanitation and water management to keep breeding sites dry. The more flies produced in an area, the more likely they are to move.
and it seems to be the answer.