Thanks, @Erekoze. That’s good to know. It appears that version upgrades (eg Oreo to Pie) are not handled OTA, so I’ll follow the wiki instructions. I hope this will not require wiping the phone … deep breath 
I’m a bit confused about the exact process to upgrade. This thread indicates the need to upgrade Mi A1 firmware, and flash a stock Pie ROM prior to installing LineageOS/Pie. The wiki mentions none of this and I do not know whether the prior requirements have been superseded by a newer ROM version.
I’ll be using the ROM image mentioned in this thread unless you can suggest a more suitable one.
Thanks for your help with this. You have saved me a lot of time and frustration!