OK, so the BIOS update did something to fix the AHCI error.
Thanks morkhans for the update.
Once I updated the BIOS and reset the BIOS to defaults it then booted into windows.
I'm still getting an AHCI error on port 0 (I'm not wondering if it's on the hard drive itself), but at least it boots.
I also installed another of the new Power Supplies into another Microserver and it words perfectly.
So the verdict is - this PSU is the one to get as a replacement...
http://www.comx-computers.co.za/FSP...Energy-Star-5-0-SFX-2-1-Power-Buy-p-77878.php