We're working out what we think - we'll add our view here!
Mountain Equipment says:
The 'gran turismo' of the Helium range. Wider and longer and offering exceptional comfort and performance for long-distance backpacking and trekking.
*Mountain Equipment use their own 'Good Nights Sleep Guaranteed' rating which they believe is a more accurate number for some sleeping bags and is based not just on the EN test results but on lab testing, their own use, thermal imaging and cold chamber work, along with a lot of data analysis.