So I tried using the Windows Phone Recovery Tool after reading a post that restoring that way resolved the error...
Big mistake.
I now have a bricked 520 showing the Nokia red screen of death. The recovery tool software is now saying it can't help as the product code is empty. Tried Lumia Software Recovery Tool but that won't discover the phone either.
All I can say is thank god this isn't my main device! Will pick up another budget Lumia next week and wait until the next build I think...
Windows Device Recovery Tool has an option in case your phone is not detected. I know this first hand because I got the red screen of death on my 950xl when the operation failed. A MS tech suggested I try again using a different PC. running WDRT on the second PC fixed the issue and my 950xl was recovered. I had to select the My Phone Was Not Detected option and then selected Lumia from the list.