Thanks very much for taking the time to get back to me again.
As of last night, I have managed to resolve the issue! Now this is an odd one to be sure but in short, I had changed some code in the sign in page, and got it to load. I then saw your post and added your code snippet to show any errors but couldn't find any on the page. I then updated the form_validation files and mysqli files to the latest versions and this is where the problems started. I then found that any session variables which were declared on a page were not getting passed from page to page, despite starting the session on each page. It was as if they became unset every time a page was navigated to another. However, the session variables for keeping the user signed in, were fine.
It took a lot of digging (yes some crying and praying too – I wont lie), and then I tried putting back the original form_validation and mysql files, and then everything works fine!
Now I am wondering if there are two issues at play. Firstly, the original issue may have actually been with my hosting provider and the SSL certificate but they say not. And secondly, there may be a possible conflict with the latest versions of the form_validation and mysql files, when running PHP v 7.3.33
However, I haven't had time to work out which of the files could be causing this yet. I will be working on this in more detail after Christmas but I'd be happy to add this info to the PHP versions thread if you like, and attach the slightly older versions of the files? Just let me know if you think this would useful to others using 7.3.33?
As far as getting a pro on board, this would be incredible but I do understand this may take time.
Thanks again so much for your support :-)