In reply to Sir Stefan:
A good few years ago we got the fill from two antique Blacks bags cleaned and recycled into a double-bed duvet by a local dressmaking/repair/etc shop that offered it as a standard service at modest cost. It's still keeping us warm.
If you can't find someone to do that, dog and cat rescue charities are happy to take old duvets for bedding, and I'm sure they'd be delighted to take your sleeping bag too, if that doesn't seem too sad a fate ...