Got some help on the forum;
downloaded asus firmware, unpacked blob, fastboot flash staging blob; fastboot reboot
Prime is now back to fully operational!
Many thanks to @kkdm
I have looked around the various threads (including the 'how to unbrick your prime' one) but have not seen this issue mentioned.
I have a strange issue where if I factory reset (i.e. to Asus) the device will simply keep looping at the ASUS EEE logo, with device is unlocked in the top left hand corner. If however I install CM10 or another MOD, it seems to work as normal and boot into the homescren.
Having read around, there was an issue with an Asus update where it appeared to do something similar to this, is it perhaps that I am on that version and thus am getting the same error? Would a simple download and copy of the latest ASUS ROM fix that? I am unsure on how to do this exactly, but have read this was an issue, but I find it unlikely as everything was fine before I unlocked etc.
Is it perhaps the bootloader has become corrupt in some way with the ICS/JB difference? This is just a guess on my limit knowledge...
The reason for putting back the ASUS stock ROM is selling the device, having upgraded to the Infinity, and do not particularly want to send it with CM10 on.
Any help would be greatly appreciated,