Default Help Resetting Surface

I've agreed to sell my Surface RT but I'm struggling to reset it. I deleted the backup partition and made a backup onto a USB drive but I've since upgraded to 8.1.

Now when I try to reset, it says files are missing from the recovery drive. I've tried booting off the USB stick, entering the BitLocker key and can't reset that way either.

Am I completely stuck now?

Edit: Nevermind, found an official recovery link at