#NAME Error with Function Defined with Public Function

Hi All.

I've defined a function to display the Category from properties as follows:

Public Function Version() As String
   Application.Volatile
   Version = _
       ThisWorkbook.BuiltinDocumentProperties("Category")
End Function

I've put a formula into a worksheet to display this value (in this case the version number) which is

="Pricing Tool "&Version()

This works fine on my laptop, however if anyone else views the file they get a #NAME error in the cell where the formula is. I was thinking that perhaps the code needed to be loaded when the file opens, can anyone advise how this would be done as using the public function with workbook_open doesn't seem to work. Ant other suggestions also welcome of course.

Thanks

Simon