Hello Ian,
Try this
//ExportPackageXMI (string PackageGUID, enumXMIType XMIType, long DiagramXML, long DiagramImage, long FormatXML, long UseDTD, string FileName)
Repository.GetProjectInterface().ExportPackageXMI(pkgGUID, EA.EnumXMIType.xmiEADefault, 1, 4, 1, 0, outputpath);
Hope this helps.
BR
Nabil