Not a hens nest

Mr S found todays blip by the letterbox. A very tiny nest about 7cm ie 2 and a bit inches across. It must have blown down from the silky ash on the street a short distance away. It has a flat bottom almost as if it has been sitting on the square street lamp above the box. But that doesn't seem possible unless the bird had fastened it to the lamp somehow, out of sight. From the bird book it is possible it is a fantails which has "a distinctive tail". There is a tail to this although squashed flat underneath now. If you can tell me any different please let me know.
Mr S thought putting a hens egg next to it would show the size better. Good thinking blip supporter.Plus, I photographed it were it lay.