Robin Bowes wrote:

Well, at the risk of having a complete conversation with myself, I've
created partitions of type "fd" on each disk and re-created the array
out of the partitions instead of the whole disk.

mdadm --create /dev/md2 --auto=yes --raid-devices=8 --level=6 /dev/sdc1
/dev/sdd1 /dev/sde1 /dev/sdf1 /dev/sdg1 /dev/sdh1 /dev/sdi1 /dev/sdj1

I'm hoping this will enable the array to be auto-detected and started at
boot.

That seemed to do the trick.

I left the array to build overnight and rebooted this morning, and
/dev/md2 started normally.

R.