I ran it a few times. It reported a potentially corrupted installation which it could not fix. However, after a couple of rounds of fix and restart, the Silverlight update is no longer available.
Is it time to celebrate?
When I try to uninstall is says: "The installer has encountered an unexpected error instaling this package. This may indicate a problem with the package. The error code is 2753."
It worked! Incredible.
The only slightly curious thing is now there is an optional update for Silverlight (KB4023307) which apparently successfully installs but then always returns.