Using the cron facility
I have got it sorted.  I had made the following errors:
1. The command line doesn't need the parameter "task", so only the first three.  This worked when I used that amended URL as a url in a browser but then I had to include it in the control panel of the hosting site.
2. For the cron job facility provided by the hosting site in the control panel, I had to change the url to:
home/public-html/components/com_civicrm...etc. 
Then it all worked well!