by Mal Grey » Wed Aug 19, 2015 10:44 pm
As said, to maximise warmth you'll want a reasonably snug fit. I can't answer your length question, I suspect you are indeed on the edge of needing a longer bag for comfort.
An import thing to remember is to ensure you go to bed warm, after all, it is YOU that heats up the bag. Do a little activity before getting into the bag, some exercises or similar can help. Make sure you've eaten enough to fuel your body through the night. And as others say, use a good, warm, mat underneath you.
You don't mention what conditions you will be camping in. If its summer only, even on the summits, the 900 should be more than warm enough. Its really designed for colder weather, and for many people it would be too warm for summer use in the UK.
A similar alternative is the Alpkit SkyeHigh900, which is about as warm. I've just got one of their Pipedream 250s - quality is excellent, and it packs down very small, but I haven't used it yet, and its more of a spring-autumn bag.