[Nexus 5X Q&A, Help & Troubleshooting] BloodBaz:
[MENTION=301158]Jacquestrapp[/MENTION], thank you for your replies. You've been a life line.

After a couple of hours I think I've now managed to do it. I've tried two laptops, multiple USB ports and two USB cables. Whatever is happening, the PC/Phone connection seems very tempremental and I have to unplug and replug in the USB cable after each reboot. However, as you say, the fastboot commands did work after I figured this out, and from the fastboot screen. Also, whenever the phone rebooted or I disconnected the USB cable, the error message was as per attached screenshot.

So, I ran the following commands:

C:\android\platform-tools>adb devices -l
List of devices attached
00cad250a0599972 recovery product:omni_bullhead model:Nexus_5X device:bullhead transport_id:5

C:\android\platform-tools>fastboot flash bootloader C:\android\images\bootloader-bullhead-bhz31b.img
< waiting for any device >
Sending 'bootloader' (4620 KB) OKAY [ 0.212s]
Writing 'bootloader' OKAY [ 0.238s]
Finished. Total time: 0.493s

C:\android\platform-tools>fastboot reboot-bootloader
rebooting into bootloader OKAY [ 0.009s]
Finished. Total time: 0.011s

C:\android\platform-tools>fastboot flash radio C:\android\images\radio-bullhead-m8994f-2.6.41.5.01.img
< waiting for any device >
Sending 'radio' (56630 KB) OKAY [ 1.633s]
Writing 'radio' OKAY [ 0.543s]
Finished. Total time: 2.300s

C:\android\platform-tools>fastboot reboot-bootloader
rebooting into bootloader OKAY [ 0.013s]
Finished. Total time: 0.014s

C:\android\platform-tools>fastboot flash boot C:\android\images\boot.img
< waiting for any device >
Sending 'boot' (11785 KB) OKAY [ 0.538s]
Writing 'boot' OKAY [ 0.133s]
Finished. Total time: 0.786s

C:\android\platform-tools>
C:\android\platform-tools>fastboot flash recovery C:\android\images\recovery.img
Sending 'recovery' (17429 KB) OKAY [ 0.545s]
Writing 'recovery' OKAY [ 0.309s]
Finished. Total time: 0.900s

C:\android\platform-tools>fastboot flash system C:\android\images\system.img
Sending sparse 'system' 1/4 (508768 KB) OKAY [ 13.739s]
Writing sparse 'system' 1/4 OKAY [ 9.283s]
Sending sparse 'system' 2/4 (524230 KB) OKAY [ 14.787s]
Writing sparse 'system' 2/4 OKAY [ 8.650s]
Sending sparse 'system' 3/4 (501070 KB) OKAY [ 14.592s]
Writing sparse 'system' 3/4 OKAY [ 12.445s]
Sending sparse 'system' 4/4 (421469 KB) OKAY [ 11.842s]
Writing sparse 'system' 4/4 OKAY [ 16.174s]
Finished. Total time: 112.698s

C:\android\platform-tools>fastboot flash vendor C:\android\images\vendor.img
Sending 'vendor' (190324 KB) OKAY [ 5.424s]
Writing 'vendor' OKAY [ 5.682s]
Finished. Total time: 11.145s

C:\android\platform-tools>fastboot reboot
Rebooting
Finished. Total time: 0.012s

The pre-google animation too ages and eventually talked about corruption on the filesystem. This is presumably because it had Paranoid Android installed on it previously. So I took the option to wipe everything (I don't have anything precious on the phone that isn't already backed up) and I've got myself back to stock Google and I'm very happy! :)

Thanks again for your support [MENTION=301158]Jacquestrapp[/MENTION].

The only thing I haven't done yet is relock the bootloader.

The guide states this warning:

"Important: do not relock your bootloader unless your phone is fully stock. You must flash the factory images first in order to remove root and custom recovery (if you have them), if this precaution isn't taken you may end up with a brick. Please see section 10 of this guide for full instructions on how to return to stock"

Now that I am booting up fine in standard Google Android 8.1, is it okay for me to issue the fastboot oem lock command? I'm not going to kill my phone am I? If it's a "yes", then I'll probably leave it a few days before I actually do it.

Thanks