Visio Workflow diagram Permissions issues

"Visio Services was unable to load this Web Drawing because you do not have the permissions to view it. To resolve this issue, ask your system administrator to grant you permissions to view it."

Looking through the logs, I can see one two pertinent errors:

Access denied. A minimum of read permission to the file http://xxx/pws/_catalogs/wfpub/Collect Feedback - SharePoint 2010/ReviewFeedback_1033.vdw is required to access it

Failed to generate Vector diagram for file http://xxx/pws/_catalogs/wfpub/Collect Feedback - SharePoint 2010/ReviewFeedback_1033.vdw Error : System.ServiceModel.FaultException`1[Microsoft.Office.Visio.Server.GraphicsServer.VisioGraphicsServiceFault]: The creator of this fault did not specify a Reason. (Fault Detail is equal to Microsoft.Office.Visio.Server.GraphicsServer.VisioGraphicsServiceFault).

Clearly this is a permissions error, because it doesn't appear for admin users, but my question is where do I set the permissions! I had a look in the Service Application page for Visio Services, but other than specifying trusted locations and external authentication (this, I believe, counts as internal but happy to be corrected)I couldn't see anything of relevence.

Thanks,
Rob