Sorting Groups

I am having a bit of trouble with the sorting of grouped data.

I have a data set that returns sales data for stock items between a user specified date range. As a result I have to group by the stock items (description or code depending on a parameter value) to prevent displaying duplicate stock items and to get aggregate sales data. However I also need to be able to sort this group by three other aggregates but have not been successful. I have done the following:

The group expression is as follows:

Function Group4(fields As Fields) As String
Dim Group As String
If( Report.Parameters!ReportSetting_ReportType.Value <> "1" AndAlso Report.Parameters!ReportSetting_DetailSummary.Value = "1" ) Then
Group = fields!Group2Desc.Value
Else
If Report.Parameters!ReportSetting_SortType.Value = "1" Then
Group = fields!StockDesc.Value
Else
Group = fields!StockCode.Value
End If
End If
Return Group
End Function

The sort expression for this group is as follows:
=Sum(Code.SortExpressionNumeric(Fields), "table1_Group4")

where SortExpressionNumeric is as follows:

Function SortExpressionNumeric(fields As Fields) As Decimal
Select Case Report.Parameters!ReportSetting_SortType.Value
Case "3"
Return fields!TillSalesValue.Value
Case "4"
Return fields!TillSalesQty.Value
Case "5"
Return GrossProfit(fields)
Case Else
Return 0
End Select
End Function

where GrossProfit is another function that returns a decimal value.

When I preview my report with ReportSetting_SortType = 1 or 2 everything displays as expected. However when I set ReportSetting_SortType = 3,4 or 5 the data does not sort how I need it to.

Can anybody see what I might be doing wrong?