Thanks both for your replies.
Ah yes JobAndKnock you must be right about it being an old Hessian sack around the pipe, as there's another sack loose around another. Just like the sacks I've seen full of coffee beans in trendy cafes.
What got me worried about the loft, having previously not...
I rent the top floor of a converted Victorian house. Over the years I've been in the unconverted loft quite a bit to store things. Then over lockdown I spent quite a few hours clearing out all the old junk that was up there. And more recently putting a bucket under a cracked roof tile...