Motley, I think the parka as used by the RAF was never used as intended. I think it was actualy the top half of a sleeping bag assembly. Rather than a full length sleeping bag which could actually be quite difficult to get in - bum shuffle and all that - it was one that would just cover the lower body perhaps up to the thighs. It wasn't intended as a working coat.
I was at Goose once with the temp in the 'high' 20s (near -30) and was quite warm in the standard Mk 3, acrylan pile etc, provided I kept turning to pick up gammas from a weak sun. As soon as part of my body was in the shade it soon became a little cool when standing around.