The MS tech guy got back to me promptly at 9.00am to check whether his second attempt to install the update had succeeded.
I advised him that it hadn't, gave him the error message I had received which was that the install had failed during the install phase and that the installation (software) needed to be updated. He had looked at that prior to his second attempt. After checking out the message and error code he advised me that he would have to escalate the problem and we arranged another call for 10am tomorrow to remedy the problem. So it looks as if I might have hit a wider systemic problem with this update.
The PC was automatically returned to the 1904 OS so no problems there.
I am,howver, impressed by the quality of service I have received from the MS Support service even if the problem has not been resolved yet and I'll update tomorrow.