Ive booted both the 8 inch and 7 inch from usb sticks
With the 8 inch - you need to use rufus to create a gpt partitioned uefi boot disk (fat32) and then with the tablet off press volume up and power until you see the boot menu.
With the 7 inch - you need to use rufus to create a gpt partitioned uefi boot disk (fat32) but from then on you need a keyboard connected to boot with and hit del to enter the bios, select the usb stick as boot override device and away you go - this does mean you need a y cable to have both the keyboard and usb stick connected at boot though.
Thanks Andy, still no luck though,
I built a Rufus gpt uefi boot stick (booting to DOS).
On the 10 inch I tried;
- setting the BIOS boot order with the USB stick first
- overriding the BIOS boot device to the USB stick
- Booting to USB recovery device from within Windows 8.1
Feels like I'm doing something silly wrong, but for the life of me I can't see what!
(The USB stick boots fine on a laptop)