I have re-run the DISM, which completed successfully, and the SFC which found no integrity violations but, at the last startup, the welcome screen displayed for 26 seconds!!!! It is worse than before!
My computer is set to shut down when I close the lid and/or when I press the power button. in both cases the welcome screen displays for 25 seconds at the next start up. However, if I click Start > Power > Shut down, the welcome screen displays for only 3 seconds (also the NUM Lock key remains "on"). This absolutely weird!!
I will try the repair/upgrade of Windows 10 in due course.
Interestingly the monthly update of Windows (which happened this morning) took less than an hour whereas previously it took 2½ - 3 hours!