I bought this at the weekend as the weather has been so chilly, and I am off to Sctoland at the weekend, I wanted something to keep me toasty. I tried on both S and M, as I am between a 10 and 12 depending on fit etc. The S was perfect for me with a medium jumper on, the M was too big, but if you intend on wearing a BIG jumper, or more than a couple of layers then this would be better for you. The description is a little misleading, the hood isn't detatchable, but the fur trimming is (it may just be me that read it as the hood being detatchable).
I loved the coat so much I wore it out of the shop, and have had lots of envious comments. Would definitly recommend this, it is a little expensive, but I will get many years of wear out of this, and what price warmth!