Setting Column Value In Script Task

This should be simple. I have a package which reads a flat file into a data flow. One of the columns (RefNumber) needs to be parsed and split into 2 distinct values. So in the dataflow I add the 2 new columns (ID1 & ID2) in a derived column transformation, and then call a script task.

In the script task, RefNumber is readonly, ID1 & ID2 are readwrite.

Here's a cut-down version of the script task with the boring stuff removed;

Public Overrides Sub Parser_ProcessInputRow(ByVal Row As ParserBuffer)
Dim narrative As String = Row.RefNumber.Trim()

If (String.IsNullOrEmpty(narrative)) Then
Return
End If

'lots of stuff happening here not relevant to the question so snipped

If ((IsNumeric(narrative)) And (narrative.Length = 16)) Then
Row.ID1 = Int32.Parse(narrative.Substring(0, 8))
Row.ID2 = Int32.Parse(narrative.Substring(8))
Return
End If
End Sub


Looking at a data viewer after the script task, the values aren't being set. I also stuck some MsgBoxes into the script task and the Row values are being properly in the script.

I know I'm missing something obvious ... any ideas?

Thanks,

Greg.