@phone-adventures replying to my own reply (newbie thing to do I know) anyway SUCCESS!
Thanks in large part to the folks on XDA. I ended up downloading 0.8.8-beta (but maybe a later version would also have worked). It displayed the exact same issue. Searching revealed a post on the LineageOS site (I think it was) with people that had an identical issue.
The advice which was followed related to data in the user partition which causes this. So the advice was to wipe all user data (i.e. check that box). I followed that advice and BINGO everything just worked as documented.
I hope this helps someone with a Xiaomi Poco M3. I've added some stuff to the web page, but more needs to be done on that.
I'll now try to figure out how to close this issue off on this forum.
