Apologies, found the issue and fix for this, it seems to only effect emonPi v1 with a RaspberryPi 3 B+ and not the emonPi2 for some reason.
After flashing the image, edit /boot/cmdline.txt and remove the first part "console=serial0,115200 "
console=serial0,115200 console=tty1 root=PARTUUID=567d5380-02 rootfstype=ext4 fsck.repair=yes rootwait sysrq=0
Replace with:
console=tty1 root=PARTUUID=567d5380-02 rootfstype=ext4 fsck.repair=yes rootwait
It will then boot fine.