Update!
I contacted Yoast who said they would only talk to me if I paid for the premium licence.
Not the answer that a prospective customer likes to hear.
I have already paid for other Yoast Premium products but not on this particular site yet.
So, left to my own devices,
I disabled The Sitemaps feature on my Yoast installation and deleted the Yoast generated sitemap from Google Webmaster Tools? I then down loaded Google Sitemaps for WordPress plug in and used this to generate a new sitemap which I then listed in webmaster tools.
Bingo! The new sitemap was processed and indexed almost immediately by Google.
This leads me to believe that there is a problem with Yoast which I am now considering removing from my site altogether after a very frustrating two weeks of troubleshooting.
Very glad Now I didn’t buy the Yoast 5 site licence!
