Datastore Backup to storage bucket failing due to incorrect permissions

[DJNinNZ]

Hi there

I'm trying to get the datastore backup to a storage bucket working but it keeps failing with 

I'm defining the bucket in the Web App as configuration as simply gng_loaner_xxxx_xxxx_backups

I have given the Appengine default service account  full storage admin permissions but the cron job to perform the backup still fails with

"error": {

"code": 403,

"message": "The caller does not have permission",

"status": "PERMISSION_DENIED"

Looking in the logs I see Attempting to export cloud datastore to bucket u'gng_loaner_xxxx_xxxx_backups'

not sure why I see the u before the bucket name

I created the storage bucket manually through the cloud console

The instructions at Datastore Backups mention a service account so I have assumed the Appengine default service account, is that correct?

Regards

Duncan