Cynic wrote :
I recall once opening a loft hatch in one house I worked in. Everything was
coated with flies. Apparently it was a cluster fly infestation.
One source suggested it was a sign of a healthy earthworm population in the
lawn.
https://www.rentokil.co.uk/blog/the-...cluster-flies/

The warmth yesterday afternoon, brought out lots of flies here in the
garden. A slim bodied fly with what looked like four quite slim wings,
which folded tight above their thorax once they landed.

I wonder what they were - is there a fly identification website?