They are made by Jane our CP fundraising lady. I found a stash of them I'd hidden and forgot about while cleaning that week so popped a couple in the parcel for Dave and Chilli.
Back in the days before we had Chester, Jane posted me a jiffi bag stuffed them. When I got home from work, I opened the door and it was carnage all over the floor.
They were bits of shreaded jiffi bag and mice everywhere.
Jane says she 'can't remember' where she gets her catnip from but I think she's just protecting her sources.