Marshal.GetActiveObject COM run time exception HRESULT 0x800401E3

I'm trying to get a hold of the excel application that is open. I'm using the Marshal.GetActiveObject("Excel.Application") method to do this. I open the excel workbook manually and then try and get a hold of it. There is only one instance of excel open so I'm not sure why I'm getting a COMException saying Operation unavailable (Exception from HRESULT: 0x800401E3 (MK_E_UNAVAILABLE))

My code is as follows

        Microsoft.Office.Interop.Excel.Application oExcelApp=null;
        oExcelApp = System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application") as Excel.Application;

Craig

I use:

    Microsoft.Office.Interop.Excel.Application oExcelApp = null;
    try
    {
       oExcelapp = (Microsoft.Office.Interop.Excel.Application)System.Runtime.InteropServices.Marshal.GetActiveObject("Excel.Application");
    }
    catch {} // can do intelligent stuff here    
    if (oExcelApp == null)
    { Console.WriteLine("Excel not found"); }
    else
    {Console.WriteLine(oExcelApp.ActiveWorkbook.FullName);}