Just following up on my post above. I've just noticed that it appears that the homepage not loading and producing a 404 appears to be when SEO URL's is installed and nothing to do with PHP 5.6 or 7.1
This is what I found in the debug files. I have three duplicate sites with exactly the same template set, the only difference is that CEON SEO URL's is not installed and they are the same colour with different content.
Code:
[04-Sep-2018 16:53:54 UTC] Request URI: /~MYWEBSITEDIRECTORY/, IP address: 00.00.00.000
#1 preg_match() called at [/home/MYWEBSITEDIRECTORY/public_html/includes/functions/functions_general.php:1031]
#2 zen_back_link() called at [/home/MYWEBSITEDIRECTORY/public_html/includes/templates/template_default/templates/tpl_page_not_found_default.php:54]
#3 require(/home/MYWEBSITEDIRECTORY/public_html/includes/templates/template_default/templates/tpl_page_not_found_default.php) called at [/home/MYWEBSITEDIRECTORY/public_html/includes/templates/venturezen/common/tpl_main_page.php:179]
#4 require(/home/MYWEBSITEDIRECTORY/public_html/includes/templates/venturezen/common/tpl_main_page.php) called at [/home/vgd/public_html/index.php:97]
[04-Sep-2018 16:53:54 UTC] PHP Warning: preg_match(): Unknown modifier 'v' in /home/MYWEBSITEDIRECTORY/public_html/includes/functions/functions_general.php on line 1031