More on this, found a bug in the code.
Line 163 of amazon-s3-and-cloudfront.php an exception getting caught:
AWS Error Code: PermanentRedirect, Status Code: 301, AWS Request ID: ##, AWS Error Type: client, AWS Error Message: The bucket you are attempting to access must be addressed using the specified endpoint.
To fix this I needed to put my ‘region’ into the instantiation of get_client() in /amazon-web-services/classes/amazon-web-services.php