I have managed to fix the problem and thought I would share that here in case can help others.
My solution was via the Siteground host so limited in relevance.

In Cpanel, I used the "Joomla Toolkit" / "manage domain" / "change domain".
It showed the current and correct domain and when I clicked on that, it permitted JSitemap to work correctly.

Note however, that I have previously changed all the URLS in the meta info table as per instructions above and dont know if that is also needed

Thread is now closed.
Many thanks for your help/.