Impatiently I crumbled and bought the mammut bag, Rock+Run compared it to the Rab neutrino range, I've used a rab neutrino 600 for the past 2 seasons and thats been brilliant so fingers crossed the mammut bag is as good if not better!
N.B if anyones in the market for a new sleeping bag the Mammut Sphere UL Winter is down from 475 to 199 on Rock+Run, just follow the link I posted above! Joe browns have about £90 off the rab bag down from 250 for 165 as well
Post edited at 14:34