I once survived -20°C in that british army arctic sleeping bag but it was a bit cold for me (with bivi bag). It's comfortable downto -10°C for me. But maybe because that bag is a bit too wide for me (I am tall and quite thin). And it's quite bulky, even when compressed - it takes much space in my bergen. But it was worth the money (bought it very cheap on eBay).
Here is a picture of the sleeping bag I am talking about: (on the left)
http://www.surplusandadventure.com/ishop/images/800/sleep_system_comp.jpg
Here is a picture of the sleeping bag I am talking about: (on the left)
http://www.surplusandadventure.com/ishop/images/800/sleep_system_comp.jpg