i have written some formula which uses a module but when i attempt to drag this formula it wont change?
is there a way of doing this?

formula is as follows

=unique(January!$A$2:$A$198,1)
i need to be able to drag this down so as the 1 changes to 2 and so on..
and the module is 

Function UNIQUE(InputRange As Range, ItemNo As Long) As Variant
    Dim cl As Range, cUnique As New Collection, cValue As Variant
    Application.Volatile
    On Error Resume Next
    For Each cl In InputRange
        If cl.Formula <> "" Then
            cUnique.Add cl.Value, CStr(cl.Value)
        End If
    Next cl
    UNIQUE = ""
    If ItemNo = 0 Then
        UNIQUE = cUnique.Count
    Else
        If ItemNo <= cUnique.Count Then
            UNIQUE = cUnique(ItemNo)
        End If
    End If
    On Error GoTo 0
End Function