Hi Phil,
If you add the following macro to, say, Word's 'Normal' template, running it will tell you whether the active document is a mailmerge main document and, if so, where it thinks the data source should be.
Code:
Sub FindmergeSource()
With ActiveDocument.MailMerge
If .MainDocumentType <> wdNotAMergeDocument Then
MsgBox .DataSource.Name
Else
MsgBox "Not A Merge Document"
End If
End With
End Sub
For help on installing/using macros, see:
http://www.gmayor.com/installing_macro.htm