Quickly creating an issue for this; proper issue summary will follow.
https://www.drupal.org/docs/8/upgrade/upgrade-using-web-browser#pre-upgr...
If the database credentials to your source database are correct, the upgrade review page will appear as illustrated in the screenshot below. This page shows a summary of the upgrade status for all installed modules on the old site.
Latest iterations use this help text instead:
The new site is missing modules corresponding to the old site's modules. Unless they are installed prior to the upgrade, configuration and/or content needed by them will not be available on your new site. Read the checklist to help decide what to do.

Recent comments
I have had a read of what Lando provides, and it's an excellent development tool, and it would solve the problem. But, I would prefer to find out why the problem is there in the first...
How often is the settings.php loaded.
Thanks for the info. I will give it a try...
How often is the settings.php loaded.
I use Lando (a wrapper for docker) and find settings changes happen straight away
How often is the settings.php loaded.