I have read the article and unfortunately it hasn't provided an answer to the issue...
We have tried all three authentication methods, none work for refreshing the extract from Tableau server.
We also looked at the sharepoint connection logs.
And we can see Tableau Server is connecting to the sharepoint site/list, but Tableau Server keeps returning an error:
When I look at the connection details it just says 'could not connect to server', not sure which server it means, Sharepoint or Tableau.
The problem is resolved. The Tableau Server was missing the Cdata ODBC driver.... Duh...
Thanks for your time anyway.