papys and saguaros Thank you for your support.
I had a big struggle to get a clean upgrade to Joomla 4.3, from 4.29 but have now overcome those issues.
Unfortunately the 'Expired' error still persists on the website after the upgrade to 4.3 and then after applying the default.php file from Joomla 4.1.5 or 4.1.4 as suggested. These files have 421 lines of code.
Visually the error on the website is:
I have set '_content' table field 'publish_down' to 'NULL' for all articles
If I create a new test article it still shows the article as 'Expired'!
Any thoughts?