In reply to Puppy mouse:
I thought about getting a S2S thermolite reactor but I was put off by the warmth to weight not being that great, which is a shame given their sparc sleeping bags are so insanely good.
I find it hard to believe a fleece sleeping bag liner can increase 11C, even 2 if layers of fleece.
I did buy an omm core sleeping bag liner which probably adds 4c? to your bag (quoted as 0.7 tog dry or 0.4 tog when a bit wet) and is very very small and light, however it is also very very short. I'm 5' 10" (1.78m) and I find on the front it comes just below the nipples. I think it's supposed to be used in conjunction with an insulating jacket?
If you need a lot of extra warmth I'm afraid you probably need a warmer sleeping bag. Liners won't have anywhere near the same warmth to weight as a good bag. If you need an extra 11c you should get a thin liner like Rab silk neutrino sleeping bag liner, to help keep the sleeping bag clean, and put it inside a better bag, or consider wearing a synthetic "down" jacket to sleep. Sorry...
Post edited at 20:18