Update, Case Statement And Sum

I would like to update a decimal column in a temporary table based on
a set of Glcodes from another table. I search for a set of codes and
then want to sum the value for each row matching the Glcodes. The
problem is I keep getting multiple rows returned errors.

"Subquery returned more than 1 value. This is not permitted when the
subquery follows =, !=, <, <= , >, >= or when the subquery is used as
an expression.
The statement has been terminated."

This is correct as there can be many rows matching the Glcodes for
each iteration of the case statement and I need to catch them all.

I have posted some of the code below and would appreciate any help as
I'm scratching my head over this one. It's all very much work in
progress again.

Regards,

DECLARE@CostCentreNVARCHAR(3)
DECLARE@COIDNVARCHAR(3)
DECLARE@TheYearNVARCHAR(5)
DECLARE@PlorBSNVARCHAR(2)
DECLARE@BusinessUnitNVARCHAR(50)
DECLARE@BranchNVARCHAR(3)

SET@CostCentre= 'xxx'
SET@COID= 'inc'
SET@TheYear= '2004'
SET@PlorBS= 'x2'
SET@BusinessUnit= 'PBUS'
SET@Branch= ‘usa'


CREATE TABLE #SummaryTempTable (
[GLD_ACCTNG_PER] int,
[Order Num] decimal(9,2),
[Summary Description] varchar(50),
[Summary Amount] decimal(9,2)
)

INSERT INTO #SummaryTempTable VALUES(199999, 1.1, 'Tot Ext Sales',
0.0)
INSERT INTO #SummaryTempTable VALUES(199999, 1.2, 'Tot Int Sales',
0.0)
INSERT INTO #SummaryTempTable VALUES(199999, 1.3, 'Inter Mark Up',
0.0)
INSERT INTO #SummaryTempTable VALUES(199999, 2.1, 'Tot Ext Costs',
0.0)
INSERT INTO #SummaryTempTable VALUES(199999, 2.2, 'Tot Int Costs',
0.0)
INSERT INTO #SummaryTempTable VALUES(199999, 2.3, 'Inter Mark Up
Charges', 0.0)


UPDATE #SummaryTempTable
SET [Summary Amount] = (SELECT sum(CASE
WHEN ((ACT_GL_NO between '4000' and '4059') or (ACT_GL_NO between
'4065' and '4999') or (ACT_GL_NO IN ('4062','4063'))) THEN GLD_Total
WHEN (ACT_GL_NO IN ('4060','4064')) THEN GLD_Total
WHEN (ACT_GL_NO = '4061') THEN GLD_Total
WHEN ((ACT_GL_NO between '5000' and '5059') or (ACT_GL_NO between
'5065' and '5401') or (ACT_GL_NO IN ('5805','5806','5062','5063')))
THEN GLD_Total
WHEN (ACT_GL_NO IN ('5060','5064')) THEN GLD_Total
WHEN (ACT_GL_NO = '5061') THEN GLD_Total
ELSE 0
END)
FROM howco_dw_test.dbo.cubeFinancePeriod
WHERE ([coid] = @COID) AND (GLD_SSN_BRH = @Branch) AND
(GLD_ACCTNG_PER like @TheYear) AND ACT_GL_NO BETWEEN 4000 AND 9999
AND GLD_CST_CTR IN ('008','021','031','041')
GROUP BY ACT_GL_NO, GLD_ACCTNG_PER)