OK.
Thanks for your advice. Sadly, re-installing Windows has failed. Twice. Once from the onboard recovery drive, once from a downloaded installation file.
Other updates are installing ok, just not this one. It was a similar story with 22H2. A couple of months of failed installs, then, overnight, it was there. 23H2 has been failing for 7 or 8 months, and I've lost count of the attempts I've made to get it to work. ho hum!