After reading your post, I decided to give it a second/third go. This time ended up erasing and set to defaults, upgraded through web interface with the previous build (r35667). Booted up with no issues. Then moved onto the more recent build (r35681) and from there it updated and rebooted with no issues.
So thanks for posting your reply. I'm using a 3rd party rather than the OEM thumb drive, so not too sure if that had contributed to the issue. Or if it was simply a PEBKAC error.