SSRS report execution much slower than SSMS

Hi,

When the below query is executed in SSMS it takes about 25 seconds to run:

DECLARE @FINANCIALYEAR VARCHAR(4)
DECLARE @FINPERIOD VARCHAR(4)

SET @FINANCIALYEAR = '2013'
SET @FINPERIOD = '10'

SELECT
	CASE 
		WHEN MONTH(dbo.SALESTABLE.CREATEDDATE) <= 2 
			THEN YEAR(dbo.SALESTABLE.CREATEDDATE) - 1 
		WHEN MONTH(dbo.SALESTABLE.CREATEDDATE) >= 4 
			THEN YEAR(dbo.SALESTABLE.CREATEDDATE) 
		WHEN MONTH(dbo.SALESTABLE.CREATEDDATE) = 3 AND YEAR(dbo.SALESTABLE.CREATEDDATE) <= 2011 
			THEN YEAR(dbo.SALESTABLE.CREATEDDATE) 
		ELSE YEAR(dbo.SALESTABLE.CREATEDDATE) - 1
	END AS [Fin Year],
	CASE 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) <= 2012 AND MONTH(dbo.SALESTABLE.CREATEDDATE) <= 2 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) + 10 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) <= 2011 AND MONTH(dbo.SALESTABLE.CREATEDDATE) >= 3 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) - 2 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) = 2012 AND MONTH(dbo.SALESTABLE.CREATEDDATE) = 3 
			THEN 13 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) >= 2013 AND MONTH(dbo.SALESTABLE.CREATEDDATE) <= 3 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) + 9 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) >= 2012 AND MONTH(dbo.SALESTABLE.CREATEDDATE) >= 4 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) - 3 
		END AS [Period],
		dbo.SALESTABLE.SALESID AS [Sales ID],
		CONVERT(DECIMAL(12, 2), SUM(dbo.SALESLINE.LINEAMOUNT)) AS [Value],
                                dbo.SALESTABLE.SALESNAME AS [Name]
FROM dbo.SALESTABLE
INNER JOIN dbo.SALESLINE ON dbo.SALESTABLE.SALESID = dbo.SALESLINE.SALESID
WHERE (
	CASE 
		WHEN MONTH(dbo.SALESTABLE.CREATEDDATE) <= 2 
			THEN YEAR(dbo.SALESTABLE.CREATEDDATE) - 1 
		WHEN MONTH(dbo.SALESTABLE.CREATEDDATE) >= 4 
			THEN YEAR(dbo.SALESTABLE.CREATEDDATE) 
		WHEN MONTH(dbo.SALESTABLE.CREATEDDATE) = 3 AND YEAR(dbo.SALESTABLE.CREATEDDATE) <= 2011 
			THEN YEAR(dbo.SALESTABLE.CREATEDDATE) 
		ELSE YEAR(dbo.SALESTABLE.CREATEDDATE) - 1 END) IN (@FINANCIALYEAR)
	AND 
		(CASE 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) <= 2012 AND MONTH(dbo.SALESTABLE.CREATEDDATE) <= 2 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) + 10 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) <= 2011 AND MONTH(dbo.SALESTABLE.CREATEDDATE) >= 3 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) - 2 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) = 2012 AND MONTH(dbo.SALESTABLE.CREATEDDATE) = 3 
			THEN 13 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) >= 2013 AND MONTH(dbo.SALESTABLE.CREATEDDATE) <= 3 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) + 9 
		WHEN YEAR(dbo.SALESTABLE.CREATEDDATE) >= 2012 AND MONTH(dbo.SALESTABLE.CREATEDDATE) >= 4 
			THEN MONTH(dbo.SALESTABLE.CREATEDDATE) - 3 
		END) IN (@FINPERIOD)
	AND GBORDER = '1'
	AND SALESTABLE.SALESTYPE = '3'
GROUP BY MONTH(dbo.SALESTABLE.CREATEDDATE), YEAR(dbo.SALESTABLE.CREATEDDATE), dbo.SALESTABLE.SALESID, dbo.SALESTABLE.SALESNAME
ORDER BY 1, 2 ASC

When the same query is executed as part of an SSRS report it takes over 10 minutes. In the SSRS report it is run when a particular financial period is selected on a different report page. It gets its parameters from that selection.Is there any way of finding out why it takes so long in SSRS?

Cheers

Paul