Hi Hugh,
An alternative to full sleeping bags is the quilt.
The idea behind them is that when you consider that the back of the sleeping bag is compressed due to lying on it and offers no insulation, then why have such a reverse and not just remove it? Saves on weight and cost ultimately.
These guys from US have excellent reviews from US thru hikers giving great warmth for less weight.
Aarrggh! As a "new user" I'm prohibited from posting the URL?!? WT heck
Search for
"enlightened equipment" using your favourite search engine!
For example, their "Revelation" quilt filled with 900 fill water repellent down for regular 6ft length and 54" width weighs is rated to -12c and weights just 588 grams.
Yes
under 600 grams!
Cost is also quoted at £255.89 (ex. shipping and potential import taxes one assumes) which is comparable/cheaper for similar rated bags.