Thanks to the AS400 application that reads this file in, I actually have to save it down to Excel 03 format (this is run on a machine running Office 07)...
//Save xls down to excel 03 FileFormat
wbk.SaveAs(filePath + fileName, Microsoft.Office.Interop.Excel.XlFileFormat.xlExcel8, null, null, null, null, Microsoft.Office.Interop.Excel.XlSaveAsAccessMode.xlExclusive, null, null, null, null, null);
wbk.Close(true,filePath + fileName, false);
objExcel.Quit();
Using Finally to release the COM object (objExcel for the application)...
finally
{
if (wbk != null) { Marshal.ReleaseComObject(wbk); }
if (wsheet != null) { Marshal.ReleaseComObject(wsheet); }
if (objExcel != null) { Marshal.ReleaseComObject(objExcel); }
if (curs != null) { Marshal.ReleaseComObject(curs); }
if (cursUG != null) { Marshal.ReleaseComObject(cursUG); }
}