SELECT     Stock_ClubID, StockID, Stock_UsedItem, Stock_Title, Stock_SellPrice, Stock_Cost, Stock_DisplayInShop,
SUM((CAST(((SELECT     COUNT(*) AS Expr1
                            FROM          dbo.tbl_StockControl_GroupItems
                            WHERE      (Stock_GroupItem_StockID = dbo.tbl_StockControl.StockID)) AS Quantity) as DECIMAL(5,2)) as Total) * (CAST(Stock_SellPrice as DECIMAL(5,2))) AS Total_SellPrice
FROM         dbo.tbl_StockControl

SELECT a.Stock_ClubID,
       a.StockID,
       a.Stock_UsedItem,
       a.Stock_Title,
       a.Stock_SellPrice,
       a.Stock_Cost,
       a.Stock_DisplayInShop,
       b.Expr1 * CAST(Stock_SellPrice AS DECIMAL(5, 2)) AS Total_SellPrice
FROM
	dbo.tbl_StockControl AS a
	OUTER APPLY
	(
		SELECT COUNT(*) AS Expr1
		FROM dbo.tbl_StockControl_GroupItems AS c
		WHERE  (c.Stock_GroupItem_StockID = a.StockID)
	) AS b;