Stu, Sounds like you got a Nonok Endurance 0deg same as me. I to bought this for the width. The first time I used it it was -5deg. I was on a closed cell mat at the time. Couldn't get to sleep, the more clothes I put on the colder I got, or so it seemed.
Second time out, last month, I had bought a Gelert self inflating mat, But at 50cm wide my hips and shoulders were cold as they were pressing against side of hammock with only compressed bag for insulation. I even borrowed a reindeer skin the second night and eventually put that under my shoulders, but it moved in the night.
So after some thought have decided to see if I could get Snugpak to make a underblanket for hammock along similar lines to Kick ***quilts one. At the moment this is ongoing. See group buy. This won't move in night. will have plenty of insulation to keep me warm in winter and if it is a bit too warm in summer just leave sleeping bag open or even do without. This is the theory anyway.
Ian.