A win7 and Win10 box both had not updated for several months now. I tried various registry and other fixes, including the Windows Update Automated Troubleshooter.
Nothing worked until on a whim I disabled IPv6 in the fritzbox. Immediately windows update starts downloading, and all is now fine.
IPv6 works perfectly well in other situations, just not here. Hopefully this will be useful for others if they have a similar problem. For reference the Win10 error code was 0x80240442