I was suffering from a hard drive I/O problem. A quick check with smartctl -A -l error revealed 38 errors on sda, so it had to be replaced.
So this morning the hard drive was swapped for a new blank one and I began the process of rebuilding the RAID.
I booted of a Gentoo Minimal Install CD. These are notable as they are the only install CDs I’ve found that contain the necessary md[0–3] devices in /dev.
. Armed with a Gentoo Install CD, I set about rescuing my system: