The Natural Philosopher wrote:
We have the same. I left the loft hatch open last night, and while I was
sitting on the bog, a wren flew in, took one look at me and flew out..

Opened a window and its gone for now. Could swear I had it all rodent
proofed to the hilt too..

Well there's your problem.

You rodent proofed it to the hilt, but completely forgot about the
hazards of wrens, completely different kettle of fish/animal/bird

Velvet