Andrewm, the solution I was told to do was to boot the tablet by plugging it in instead of via the power button. To do that, you simply do the exact same steps (IE, hold the hold the window button) and then plug in the power. I haven't had anyone so far say that having it on mains power didn't work just as well as battery for flashing. If memory serves me correctly, they're both more or less treated the same during flashing.