I finally got around to trying out the Promise Fastrack TX2 card -
I removed the Promise RAID card & installed the Fastrack card, got everything set in bios then tried to start the system.
Even though I have the PCI order to read from LAST to FIRST and the Adaptec Raid controller is last with the Fastrack being First the Fastrack initializes before the Adaptec SCSI ctrlr.
They both init and both see their drives, the trouble is that with the Fastrack card enabled the SCSI RAID Array isn't found & wont boot.
As soon as I disable the Fastrack the SCSI array is found & boots perfectly.
This is most bizarre, I think it's likely just a fundamental incompatability & I have pretty much given up unless someone has an idea I can try that might make it work.