Help With Stored Proceedure Please

Hope someone can help.

I am trying to write a stored proceedure to display sales activity by month
and then sum all the columbs.
The problem is that our sales year starts in April and end in March.
So far I have been able to get the sales info my using to sp's, one that say
month >3 and the other says <4. I pass in a year parameter, that for this
years figures would be 2003 for sp1 and 2004 for sp4.
I am sure there is a better way.
Below is a copy of one of my sp's.
Hope you are able to help.

John

ALTER PROCEDURE dbo.sp_SalesAnalFigures_P1
(@Year nvarchar(50),

@CCode varchar(50),

@SCode varchar(50),

@OType varchar(50))

AS SELECT TOP 100 PERCENT DATEPART(mm, dbo.InvoiceHeaderTbl.InvoiceDate) AS
Month, SUM(dbo.InvoiceHeaderTbl.InvoiceTotalNet) AS Sales,

SUM(dbo.InvoiceItemsCostQry.TotalCost) AS Cost,
SUM(dbo.InvoiceHeaderTbl.InvoiceTotalNet -
dbo.InvoiceItemsCostQry.TotalCost) AS Margin,

COUNT(dbo.InvoiceHeaderTbl.InvoiceNo) AS NoOfInvoices,
AVG(dbo.InvoiceHeaderTbl.InvoiceTotalNet) AS AverageValue

FROM dbo.InvoiceHeaderTbl INNER JOIN

dbo.InvoiceItemsCostQry ON dbo.InvoiceHeaderTbl.InvoiceNo =
dbo.InvoiceItemsCostQry.InvoiceNo

WHERE (DATEPART(yyyy, dbo.InvoiceHeaderTbl.InvoiceDate) = @Year) AND
(dbo.InvoiceHeaderTbl.CompanyCode LIKE @CCode) AND

(dbo.InvoiceHeaderTbl.SalesManCode LIKE @SCode) AND
(dbo.InvoiceHeaderTbl.OrderType LIKE @OType)

GROUP BY DATEPART(mm, dbo.InvoiceHeaderTbl.InvoiceDate)

HAVING (DATEPART(mm, dbo.InvoiceHeaderTbl.InvoiceDate) > 3)

ORDER BY DATEPART(mm, dbo.InvoiceHeaderTbl.InvoiceDate)