Thanks again for the help. I am struggling a bit with the intricacies of this but am progressing a little I think.

I have not yet tried Russell's suggestion of purchasing two new 4TB drives because of the expense but will go in that direction as a last resort. I have copied the data onto 2TB drives which are the same size and type as the originals. Not sure if that will achieve all that is required.

I have been trying with mdadm to mount the drive but so far with no success.

My mdadm.conf shows the drive: 

# definitions of existing MD arrays
ARRAY /dev/md/2  metadata=1.2 UUID=b5f4346d:deb6f751:e69ac550:b0b961fe name=DiskStation:2

but when I try to mount the partition sda5 I get:

bob@Comp1:~$ sudo mount /dev/md2 /media/ntfs
mount: /media/ntfs: can't read superblock on /dev/md2.

bob@Comp1:~$ cat /proc/mdstat
Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md2 : inactive sda5[0](S)
      1948779840 blocks super 1.2
      
unused devices: <none>

Any further instructions or suggestions appreciated

Bob