I love Cos too, I am 5ft4 & size 14 with boyish figure (slim hips & no waist) but with reasonable size norks and find the fit great. I live in Yorkshire so only get down to London occasionally but ALWAYS visit a Cos store. I agree with other posters that you can never go by the sizing or how things look on the hanger, it is a case of trying things on.
I have dresses for work and tunics mainly, I am currently 22 weeks pregnant with twins and finding the loose fit very accommodating for my growing bump.
We have a weekend planned in London in 2 weeks time and I am already looking at Cos online to decide what I might still fit into. I will definitely be visiting a store while down, even if to only look. Who am I kidding?