jezreel62

In the end I had to:
Unplug and replug the power supplies on th ds4246. This resolved the shelf amber warning light.
Then none of the disk lights were illuminated, so I had to take all drives (8) from the trays and try each one in order in different trays.
Turns out there's a knack, and you have to kind of position them towards the business end of the tray, push gently then give it a good shove at the end, to get it to "connect".
Eventually all of the disks were accepted and recoginised by the shelf, and in turn by unraid.
I flashed the firmware of the LSI to p20 in the process (to eliminate that possibility).
 
So, thanks for help and looking ladies and gents, both here, and all over this board, I appreciate it.
I can't really mark anything as a solution other than "persistence" here though. What a learning curve, but what a deep satisfaction when all those drives were available to assign in unraid. No doubt there'll be more questions from me in the future. 
Peace!