It's a Microtech Ultralight(?) I think I bought it on Amazon. Comfort zone is listed down to 10 degrees celsius and that seems about right. You can go a bit lower with something like the Sea To Summit Thermolite liner listed above. But it really is for the warmer nights.
Know its been sold already, but whats the brand of the "tiny synthetic summer sleeping bag" and what kinda temps were you taking it down to?