Mail Merge 403 Error

SOLVED

Has anyone encountered a 403 error when trying to perform a mail merge to a Word template?

403 - Forbidden: Access is denied.
You do not have permission to view this directory or page using the credentials that you supplied.

It used to work but now doesn't, frustratingly.  I thought at first that it might be something to do with https but I have disabled the require SSL option in IIS and that hasn't solved the problem.  I have also explicitly set the CRM App Pool credentials to a system admin account but that hasn't helped either.  This affects all templates.  I have also checked the permissions on the template files themselves and the IUSR account has effective full control over the file.