DosDude to the rescue again, I downloaded the High Sierra patch, installed it on a USB drive, and used the patch to download High Sierra. I booted from the USB, opened Disk Utilities and re-formatted the HD, then installed High Sierra.
With HS installed, I swapped the 1Gb GFX back to the 3Gb Metal GFX card to prove it was either faulty or working - it was working! So, leave the 3Gb card in, download DosDudes Mojave Patch and Mojave Installer, then disk Utilities, Format and then install Mojave.
A few system tweaks and restarts to check and everything seemed to work. I downloaded Air Display and installed, it then asked for a restart and......
I WAS BACK WHERE IT ALL BEGAN 2 DAYS EARLIER, the Mac refused to boot past the halfway mark.
So now I knew the culprit was Air Display on this particular Mac, I had Air Display working on an almost identical Mac Pro 5.1 with Mojave and my new 2018 Mac Mini with Big Sur.
It was back to square one, only this time as I knew the 3Gb Metal GFX card was OK, I tried a HD reformat and 20 minutes later the Mac booted to a clean install of Mojave.
The next thing to try was another Mojave install and then update, it made no difference, the Mac refused to boot past the half way mark on the boot sequence.
So, after countless restarts and installs, I can have a fully working MacPro 5.1 with Mojave and without Air Display or a dead Mac.....