Update on the recovery operation.

Replacement drive arrived yesterday, BIOS chip and board swapped. Drive came up as available.

The original array was set up in Open Media Vault but I've subsequently moved to a four bay Synology so I loaded up an SSD with OMV and plugged everything in to a spare PC . OMV recognised the array without any prompting (handy to remember that) and reported, as expected, a disk was missing. As I only had one compatible board for two of the drives. No matter, 3 out of 4 disks is good enough to establish the RAID.

Simple click to mount the array and new 8Tb and copy is underway.

Big sigh of relief here.. So big thanks to all contributors.