@Brian706 got lineage installed with TWRP btw.

I'm nearly certain following the same lineage wiki instructions (i.e. with their own custom recovery) would have worked after taking a system firmware update once I had the stock ROM back. Really can't imagine why I either didn't clock or ignored the message about making sure you're on the latest 11 firmware (in a red box!) at the top of the page. Perhaps I just thought it would be fine as it was a 'fresh' phone. Lesson learned.

However I decided I wanted TWRP anyway so went that route. There were quite a few other issues to scramble over on the way. Perhaps the oddest was a worrying complaint in TWRP right at the end that there was no 'system' - after I'd wiped the stock ROM and flashed the lineage one - but it rebooted to lineageOS after repairing the user data partition anyway! - is that normal ???).

Anyway, got there. Made copious notes

 

PS now I realise why I didn't see the warning to update the firmware. At the time of attempting the first install I was (unknowingly) viewing a repost (on lineageosroms) rather than the original wiki (on wiki.lineageos.org). Beware