The outer glass of the bathroom window has a sort of 'hammered' finish and looks like this when viewed from a distance. The trees, grass and greenhouse can just be made out along with a white plastic chair near the bottom right:
When looked through from a few inches away the curvature on each indentation gives some interesting patterns:
I often see broken egg shells on the ground but this one was perched on top of the garden fence. I think, from the markings and its size (21mm), this is a House Sparrow egg. I wonder what raided the nest and rested it on the fence while it carried out the dirty deed.
Thanks again to Ragged Robin for identifying the Vapourer Moth in my last post.