druidh – Member
Small, light, warm. I hope you have the cash.
Down is smallest/lightest but often frowned upon for bivvying as it suffers when damp. If you can get around this, look for a 4-season Rab, Mountain Hardware or Mountain Equipment bag.
Well he aint gonna get any of those for £100!! 🙂 4 season might be a bit overkill as they'll be rated to -17c to -20c. You should be able to get away with 3 season and a bit of exra kit if temps do drop. 3 season are rated to -9c ish.
With your budget, I'd go for a decent synthetic bag rated to about -10c. Should be £70 – £100. Bigger and heavier than down but you'll need another ~£150 – £200 for that!