I need a new UL winter sleeping bag and as always when looking at bags, struggle with the numbers. The most extreme I camp in is higher spots in the Lakes in February. So not the arctic or high mountain peaks. The question is then just what rating bag should I be looking at? I know there are lots of factors like how cold I sleep, what I'm wearing, what mat I'm using, the tent, etc. but what's the ballpark? I have a "feeling" around a -7C (comfort rating) bag seems right but could afford a -15 bag, but is that unnecessary weight and cost? Two I was looking at today are the Sea to Summit 2019 Spark Sp III (-8C, 660g, £414) https://www.ultralightoutdoorgear.c...c88/2019-spark-spiii-down-sleeping-bag-p11308 versus their Spark IV (-15C, 856g, £489.99) https://www.ultralightoutdoorgear.c...-c88/2019-spark-spiv-down-sleeping-bag-p11309.