This thread, which I can't post in yet:
At the end of step 8, I went to Reboot/Power Off as instructed. The phone was still plugged into the PC.
For step 9, I started up flashtool on the PC. Flashtool reported "Device connected with USB debugging off". It was definitely ON when I started - I checked - as is 'allow installation from unknown sources'. Huh???
So, after disconnecting and reconnecting the USB a few times, I gave up on that and thought maybe I need to start the phone up again and re-enable USB debugging. (going against 'do not reboot' in step 8.
Then - Bootloop! So that's why we should not reboot at that stage.
Thank NUT for reminding us to do a backup first (once I remembered after ten minutes panicking that I could restore it...)
Anyway, after restoring the backup (it works, wipes sweat from brow, I checked and USB debugging is indeed still ON.
Plugged back into the PC and flashtool now identifies the Z1.
So, what is going wrong, and how do I fix it please?
P.s., yes, I'm a noob to this.