After following the instructions, I ended up with an error about non-existent service.
The service "Bref\Messenger\Sqs\SqsTransportFactory" has a dependency on a non-existent service "Aws\Sqs\SqsClient".
I've realised that I needed to configure it myself and it might help others if this was included in the readme.
Aws\Sqs\SqsClient:
arguments:
- { version: latest, region: eu-west-2, credentials: { key: xxx, secret: xxx } }
I believe that credentials is optional, if you have credentials stores in ~/.aws/credentials