Thanks for the comments so far. Stuart D - yes to all the above. I was car camping, so I was well-fed, well-watered, well everything and using my warm double-walled tent with fabric inner and my sleeping bag rated to -5 deg C. It's not new but it is clean and the reason I sleep in boxers and a t-shirt in it is otherwise I overheat! I had to keep the side zip and the toe-box zip open most of the night. So, my body wasn't cold. But even while feeling otherwise toasty warm, when I lay on my side I could feel "coolness" under my hip, which I'm concluding was either localized heat loss at that point, or my imagination.
Stuart D, when you say "the mat has been comfortable down to ~3-4C", do you mean -3 to -4 deg C, or +3 to +4 deg C?
On a hiking trip (as opposed to this car camping trip) I would be in a "colder" tent, wearing all my layers, and using a quilt. So, if I wasn't imagining things and the Synmat is borderline at the temps I normally encounter (0 to 5 deg C), should I swap it with the Downmat (add 105g), or use a bit of foam under the hips (add ?g)?
BTW, the minimum air temp was likely in the range 5 to 8 deg C with no significant wind chill. The car next to me registered 5 deg C, mine registered 8 deg C, and a thermometer at a nearby house registered 8 deg C.