Ooh, aah !! I think I've done it.
I went to "help & support", searched for "schedule task" and opened the "task scheduler".
I then "created a basic task", adding the /AUTO flag (was that necessary?). I can't recreate what I did without making a new task, but, at some point, I was offered the chance of "granting the highest privileges" or suchlike.
On rebooting the computer, I couldn't see anything happening, but getting CCleaner to analyse manually it found nothing to delete. So that looks like SUCCESS !!!!
Thanks to Lyronx and Nergal (where do you get those names from?) for your help.