I did however test the Nexus being charged on:
First: 1.0A rated microusb wall charger - worked the same as the Asus charger, in that it would charge it while the Nexus while it was off but nothing while the Nexus was turned on.
Second: 1.0A rated car cig lighter to usb socket, with the Nexus oem usb to micro usb cable. This worked properly - the Nexus showed it was being charged while it was on.
From this it seems there could be a problem with the oem wall plug?