Hey, after activating Dev Mode, and trying a couple of different USB cables, things worked out. The only other sticking point was that I had to remove -w from flash-all.sh and then the stock image installed fine. I'm super happy to have my Nexus 10 up and running again after 5 years in a box. Already getting lots of use out of it Thanks for your help. I don't think I would have found Dev Mode otherwise I'm going to update my OP and add the solution.

I can't edit my OP, I guess due t anti-spam measures...

UPDATE (26/4/2020): I resolved my issues by installing stock Android back on to the Nexus 10.

My problem with getting remote access to the Nexus 10 was a mixture of a bad USB cable and/or USB port. I also had to activate "Developer mode" on Ubuntu Touch: Settings > About Tablet > Developer Mode. Once I got a connection the command: adb reboot-bootloader worked.

I installed the stock Manta image (as below). I had to remove the -w from the flash-all.sh and then the image installed. I've got a shiny Nexus 10 to use again.

BACKGROUND

I can get into the Ubuntu Touch recovery mode, version: (CWM -based) Recovery v6.0.4.6, on the Nexus 10, using the key combination: hold down both up and down volume buttons and then press power button.

I have followed the following post but I’m stuck at the command: adb reboot-bootloader - How to restore Android.

Regarding which version of Android to restore onto the Nexus, I have downloaded a copy of stock Manta from here, which, when unzipped, includes the flash-all.sh script.

@Keneda, yea, I had such high hopes back in 2015. I bought the Nexus especially to put UT on it. I had an Ubuntu Phone at the time too. It looked like I was breaking out of the Google-sphere, only to be sucked back in by the monster