Is there any documentation on how best to set up S3 storage? 

I'm building a new site and have a SW bucket set up as "Objects can be public". 

The bucket is setup with encryption.

I've filled in the keys in my Una install but have not filled in the "AWS custom domain" field.
Is this required for this to work? If so I know I can view the AWS docs at https://docs.aws.amazon.com/AmazonS3/latest/dev/VirtualHosting.html

However, first assuming an AWS cusrom domain is not required then I try to upload an image  it won't work and I get a S3::putObject(): [AccessDenied] Access Denied error on my server.

I do auto backups to another bucket from the same server and no problems. Does the Bucket for Una need to be set up differently?

Sort of feeling my way around in the dark here a bit