If anyone else wants to take a stab at this please be aware that I discussed this issue at length with SD in private, he was unable to read any data from the eMMC drive at all. To get to the shell it was taking him 3 minutes and none of the usual power button/volume options were working (both of which I think read data from a boot section of the eMMC, although I could be wrong about that).
I came to the conclusion it was a H/W issue with the eMMC drive, but I would be genuinely happy to be proven wrong on that…