Windows 7 upgrade to to Windows 10 Pro, version 1511, 10586 constantly fails asking for reboot

My Windows 7 Pro (x64) machine has tried to do the auto upgrade to Windows 10 Pro (v 1511, 10586) but has failed.

Windows update history shows that this is repeatedly trying to install but fails each time with error Code 80240020.

Every time I boot the Windows Update popup is shown saying "Restart your computer to finish installing important updates". If I try to postpone this, it forces PC reboot in 15 mins. Only if I ignore this popup totally then I can continue working without getting forcibly rebooted.

Windows updates are provided by an inteal WSUS server, so do not have the option. Have 'declined' this upgrade in the WSUS to no effect.

How can I prevent the upgrade from trying to complete (and remain on Win 7)?