If you are rooted, running custom recovery or have made any other changes to your phone software such as using Xposed then you'll need to revert to stock or the update will fail. Stock roms can be found at http://sbf.droid-dev...e.php?device=14
~Rob: You can't update if you are rooted since it has modified the system partition. You need to revert back to stock.
Is adding the superuser apk all that has changed in /system? I might have a go at removing that later and then having another go with the OTA update...
My final goal is probably installing CM but I thought I'd give stock Kitkat a go first (with the minimal amount of complete wipes!).