My final update for the moment because I’ve run out of ideas of what else to try. I switched to HDMI/Keyboard and disabled serial per the instructions. I could see that the kernel started to boot, but then settled into a loop showing:
invalid bus width
error -22 whilst initialising SD Card
I reflashed the image to the SD Card, but that didn’t make any difference. It doesn’t appear that U-Boot is not very happy with my hardware for some reason.
Craig