"What do I require...?"
The patience of a saint is what! After 19 "Update Failed" messages in a row, I found, downloaded & ran the Windows Update Troubleshooter which fixed whatever was apparently wrong then located a Command Line instruction that has now forced the download to begin.
I live in the village of the damned as far as BT's cabinet upgrades go & I've worked out that if the update status bar is accurately showing the time per %, the download will take 2.83 hours.
Heigh-ho...