Can you tell us where this was found? 

At spring wood, leics Derbyshire border last weekend.

Suggestion from Colin Bowler - Tree Bumblebee, Bombus hypnorum, nest? This species uses bird boxes. 

I think we've solved this one, with a little help from our friends on facebook - it's probably a Bee Moth. 

I had the same thing in a bird box in the garden a few years back. Incredible strength for a mass of cocoons, I had to virtually wrench the lid off!