Update to close the post.
Problem resolved (thanks to prompts at various stages from James). It took the following steps and in the order given:
i) Batteries replaced twice with fresh ones from stock (note the originals still showed 80/90% and OK on a separate battery tester). FAILURE.
ii) Ensured meter was not near any known alternative Bluetooth source. For instance I was wearing a Garmin watch which was also synced over Bluetooth to its app on the same phone. FAILURE
iii) Carried out the upload using the legacy option. (notwithstanding phone is 6month old One+Three with latest Android/One+ firmware). SUCCESS.
At this point an uncorrupted firmware message displayed in the app bootloader. However app was still locked in bootloader mode
Resetting the meter and phone restored full functionality ie removal of the meter batteries, rebooting the phone, reinserting the batteries once the Android home screen appeared and then restarting the app once the meter red led had ceased to flash.