Okay, so now have reset this Surface 2 with the recovery image downloaded from Microsoft, which required the tablet's serial number. I would expect that to mean that the recovery image is somehow linked directly to this particular device.
Have reset 4 times now, and still cannot activate Windows RT 8.1. Ironically, Office RT 2013 is activated.
The activation screen shows:
Error code: 0xC004C003 The activation server determined the specified product key has been blocked.