We think this is the start of a wasps nest. I was in the loft a few weeks back and 2 huge wasps were up there. I sprayed them, then yesterday on doing a bit of sorting my husband discovered this. I think we had a luck escape from a wasps nest.
We currently have three in the loft. Not sure why my Dad won't do anything about them - they are on the same level as the library, and wasps eat books!