So my home server threw a drive a few days ago and has been down ever since, mostly because I've also been down but that's another story.
Rather than rush in to trying to fix it while not 100% I have, instead, spent some of my recovery time studying the subject of btrfs replace.
Today I booted up the server using puppy linux and followed the procedure on the btrfs wiki page under 'replacing failed devices'
This disk was one of a three disc RAID1.
Currently rebuilding on a spare disk 👍