SSRS 2008r2 Using Aggregate on field with an Expression Getting an Error

SSRS SQL Server 2008r2

I'm trying to perform an aggregate (Sum) on a field within a GROUPing that contains an expression.

The field where I want to SUM to appear is within a different GROUPing.

I've created the following Function within Report Properties

    
    Dim public tot_OT_Hrs As Decimal 
    
    Public Function Add_OT_Hrs(ByVal OT_Hrs As Decimal) AS Decimal 
    
    	tot_OT_Hrs = tot_OT_Hrs + OT_Hrs 
    	
    	return OT_Hrs 
    
    End Function 
    
    Public Function GetTotal() 
    	return tot_OT_Hrs 
    End Function

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I've added a call to the "Add_OT_Hrs" function in the field where the expression is and this works fine.

=Code.Add_OT_Hrs(
	IIF(Sum(cdec(Fields!HrsWorked.Value)) > cdec(Parameters!StdWorkingHrs.Value), 
		Sum(cdec(Fields!HrsWorked.Value)) - cdec(Parameters!StdWorkingHrs.Value) + Sum(cdec(Fields!Rate1Hrs.Value))
			, Sum(cdec(Fields!Rate1Hrs.Value ) )
		)
	)

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Fields!Rate1Hrs.Value is the field in which the expression resides in and Fields!HrsWorked.Value is an adjacent field.  Both are defined as DECIMAL(10,2) in the proc.

However, the field where I want to total to appear I've added the following

    =Code.GetTotal()

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and all that is returned here is 0.00 on every row in the GROUPing.  If I initialise the Dim public tot_OT_Hrs As Decimal variable to say 1.2 then 1.2 is returned on every row in the GROUPing.  The Add_OT_Hrs function isn't working as expected.


Where am I going wrong?

Thanks in advance.
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