SQL2012 - Availability Groups - backing up via replica

Hi,

We have beeen using RegGate Backup for some time and recently migrated to SQL2012 and implemented AllwaysOn Availability Groups. Until now we have been backing up directly on the "primary" read/write node however we now need to move this to one of the secondary nodes.

We have RedGate Backup 7.3 installed on each node however when we try and backup on the secondary we get:

08/06/2013 10:51:40: Backing up AManagement (full database) to:
08/06/2013 10:51:40: G:\RedGateBackups\FULL_(local)_AManagement_20130608_105140.sqb

08/06/2013 10:51:40: Error 880: BACKUP DATABASE permission denied in database: (AManagement)
08/06/2013 10:51:40: SQL error 978: The target database ('AManagement') is in an availability group and is currently accessible for connections when the application intent is set to read only. For more information about application intent, see SQL Server Books Online.

I'm not sure how to make RedGate use the ApplicationIntent=ReadOnly option, or whether there is something wrong in my current setup?

On the secondary node I've connected to both the listener IP and the local IP with the same results. The AG is setup to prefer secondary for backups.

Can you point me in the right direction?

Thanks
David