Thanks, Jerry
The specific solution you suggested did not work, but the link you provided was very helpful. For the benefit of anyone else with the same problem, the solution that worked for me was running from the elevated command prompt, as explained in a post by a user called CliveJFox. This happened shortly after one of those all too frequent Windows 10 updates, which I guess broke something, as so often seems to happen. I had done my laptop PM6 update installation before the same Win 10 update, which perhaps explains why that worked, but my desktop installation had the problem.
Graham