Initially it would boot and and then reboot just after the login prompt appeared. I think that was a process not seeing the hardware it expected and trying to recover but diagnosing this I hooked it up to the monitor to see what was going on.
The odd thing that is since then it won't restart without a monitor attached. If there is no monitor it kernel panics and says it can't it can't mount the root file system. I tried hooking up a cable with nothing on the end but that fails as well so it needs to get the information from a monitor to start.
the /boot/cmdline.txt file is
dwc_otg.lpm_enable=0 console=ttyAMA0,115200 console=tty1 root=/dev/mmcblk0p6 rootfstype=ext4 elevator=deadline root-wait
I think the problem is root-wait. This might have got auto corrected when I loaded the file onto the Mac to add init=/bin/sh to do some trouble shooting. It looks like it should be root-wait no hyphen.
Looks to be sorted now. So be careful editing that file on machines that want to auto correct things.