@Cerys J. Owen
Hi,
Please try below steps to rename SoftwareDistribution folder:
net stop wuauserv
Ren %systemroot%\SoftwareDistribution SoftwareDistribution.old
net start wuauserv
Then, re-start system in Clean Boot with 3rd party process disabled, and try to check/install update again to check the result.
How to perform a clean boot in Windows:
https://support.microsoft.com/en-us/help/929135/how-to-perform-a-clean-boot-in-windows
Hope above information can help you.
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