Ref post #53, I tried a third time from scratch, this time disabling Avast anti-virus software. The update reached 99% much more quickly (2.30pm) but then seemingly stalled. Turned the monitor off and got on with other things. Checked at bedtime, still 99% - ah, what the heck, left it running overnight. Turned the monitor on this morning and bingo - 'Welcome to Windows 10' or words to that effect.
Will now put the whole shooting match up for sale as I recently switched to an iMac. Farewell Windows!