SENDING ISSUE REPRISE
This is now a serious issue.
I decided that the earlier failure to send might have been a glitch that would not repeat itself.
I therefore recreated the mailing. In order to ensure that it was not just a timing matter of PowerMessenger thinking I had been online doing nothing too long and then auto-logging me out, I prepped everything; logged out; logged in again; and went straight to the Send function and clicked.
The progress bar duly came on screen and stepped through the emails to send out. However …
… It once again hung on '42'.
This time I only waited another 30 seconds or so before aborting and clicking stop.
If it has happened twixe in a row at the same point, then I think it fair to say this is an issue.