I am posting to say that the solution given by DatayYC (on page 2) worked perfectly for me. I was able to restart and complete the setup (Windows 10 v 1709) and all my installed software and user data was preserved (as indeed it should have been had things not gone wrong).
To follow DataYC's method you need to use installation media (USB/DVD) for the version of Windows that you were trying to update to. So if you don't have that already you need to run Microsoft's Media Creation Tool ( https://www.microsoft.com/en-gb/software-download/windows10 ) on another computer that is actually working and choose the option to create bootable media. But with bootable media the rest was easy and straightforward, as per the instructions given. One piece of advice, it might be a good idea to take a quick look at the contents of the installation media on the working computer before you start, so you recognise it again when looking for setup.exe. The drive letter assigned to it will vary from one machine to another