Hi Richard
First of all I would like to say thanks for your excellent set of instructions. They seemed daunting at first but I worked through them and eventually got there successfully - however I did deviate in a couple of areas to achieve the result.
With regard to point 3. I found I couldn't download the ISO file to the SD Card (NTFS formatted) in the D: drive due to lack of space. I followed the tip given by Stephen-C-Norris on April 5, 18 and downloaded the ISO onto the SD Card with it plugged into my desktop machine. I also extracted the file using 7-zip (WinRAR didn't work for me) while the SD Card was still in my desktop machine.
I had found previously when carrying out upgrades on my Asus E200H that it didn't recognise the Micro SD Card slot on the Asus so I fitted the SD card into a USB adapter and plugged this into the Asus (D: drive). I did this with the extracted ISO file and picked up at point 7. and ran the setup.exe file. At first it needed more space on my D: drive as I was only using a 16GB Micro SD but I deleted the compressed ISO file (~3GB) and the installation ran through (over a couple of hours - but I was having dinner!) so when I returned all I had to do was to respond to the configuration requirements and it was all done. I was running on V1803 of Windows 10.
I checked my C: drive storage and I had ~3GB of free space but I carried out a Disk Clean-up and selected the 'Clean up system files' option and checked to delete previous versions of Windows. This freed up a massive 19GB of space (I'd never been able to free up that amount previously - even when selecting the same option) so it was altogether a successful exercise. It has returned my Asus to a useful device.
Hoping my findings may be of use to others.
Charles