How about:
PHP Code: 
SELECT [fdmsaccountno], 
       [ho], 
       [rm_sales_band], 
       [rm_code], 
       [post_code], 
       CASE 
         WHEN [rm_sales_band] = '2M to 4m' THEN 'Test' 
         ELSE [rm] 
       END AS [RM], 
       [mcc_code], 
FROM  (SELECT sub.*, 
              rm 
       FROM   (SELECT[fdmsaccountno], 
                     [ho], 
                     [rm_sales_band], 
                     [rm_code], 
                     [post_code], 
                     CASE 
                       WHEN Isnumeric(RIGHT(LEFT(post_code, 2), 1)) = '0' THEN 
                       LEFT(post_code, 2) 
                       ELSE LEFT(post_code, 1) 
                     END AS 'sPostcode', 
                     [mcc_code] 
               FROM   [FDMS].[dbo].[dim_outlet] 
               WHERE  [rm_sales_band]IN ( '2M to 4m', '4m +' ) 
                      AND [ho] = 'Y' 
                      AND rm_code = 'na' 
                      AND iso_account = 'N' 
                      AND fdmsaccountno NOT IN (SELECT [ta_mid] 
                                                FROM 
                          fdms_partnerreporting.tmp.trade_assocations)) Sub 
              INNER JOIN [geo_pca_sellers] 
                      ON [pca] = spostcode)