Well, that was annoying!
The tablet came with Android 11 - first thing I checked - as required by the install instructions.
I went through the process of downloading the (appropriate) ROM and recovery files, setting up Heimdall, and turning on USB Debugging. All OK.
BUT, there was no OEM Unlock option available under Developer Options, so I followed the advice on the Install page to do a software update in order to have the option appear after an update and reboot. Yes, that worked - there was such an option after reboot.
However, the bloody thing then kept on updating a few more times, and ended up installing Android 12 on the device! 
Now I find myself in the position of having to downgrade from Android 12 to 11 again, per the install instructions which stipulate this. *sigh*
And, just to be really annoying, the OEM Unlock appears to have vanished again.
Has anyone had to do this, and if so, where can I find the instructions?
I am assuming that the e/OS/ Android S version is still a way off…