Using Amazon S3 with Snapshot

Hi, I'm struggling to get Snapshot working with Amazon S3. I'm using Amazon S3 for another feature on the site (media uploads) so have the connection details from that which are working fine. But when I put them in to the Snapshot I get the following error...

Connecting to AWS
Using SSL: Yes
Setting Region: EU (Ireland) Region (s3-eu-west-1.amazonaws.com)
Using Storage: Standard
Using ACL: private
Sending file to: Bucket: : Directory: Error: Send file failed :400 :

Do I need a separate connection key?

I'll be honest I'm a bit flummoxed by Amazon S3 setup in general and feel like I only just managed to get it working for file uploads so I may have done something wrong! Is there a walkthrough anywhere of how to setup Amazon S3?