I killed my Mac! (Part 2)
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.