We're back in business. By chance I'd mounted then copied the EFI partition (Clover Configurator) on the System drive to my Toshiba USB drive which also has an EFI partition (Apple format is good like that). With both mounted, I'd simply copied using Finder, the EFI from the System to the Toshiba.
When we replaced the Hack SSD with a PC SSD we were suddenly presented with the old Clover (4630) from the Toshiba. So in the Bios we changed the Boot Order to boot from the Toshiba. We restarted the machine and hey presto, it started Clover (4630) and Defaulted to OSX High Sierra. With the system working I mounted the System and the Toshiba and simply deleted the System EFI and then copied the EFI folder back from the Toshiba, restarted and the old version was back. Don't forget to set the boot order back.