Panic averted! I finally realised what was happening. I tried deleting the downloaded update and redownload it, but still got the failure to verify notice.
I was wracking my brain to think what I may have done differently with this update, and then I had a brainwave, yesterday morning I had invoked my Nord VPN to connect on ALL wifi connections, and when I turned it off the verification started and the installation completed perfectly [emoji122].
So there’s a warning for anyone trying to download and install ANY software updates......TURN OFF YOUR VPN!!