Are there beetles that spin cocoons?

Just saw something unusual: an insect that looked like a thin black beetle about 10mm long, hanging by a thread from the ceiling. The underside of it seemed to be covered in white fluff and it was busy doing something with its front legs, almost like a spider wrapping up a fly, but it was definitely not a spider.

Using its thread I moved it to a hedge in the garden. That didn't seem to disturb it. I had to leave then so I didn't see what it did next.

I'm intrigued. Any suggestions as to what was going on? This was in North west England by the way.