CREATE FUNCTION RetrieveMissingNumbers (@TMinVal VARCHAR(16), @TMaxVal VARCHAR(16))  -- Pass min and max tissue numbers in as parameters
RETURNS @MissingVal TABLE
 (MissingVal VARCHAR(16))
 AS 
BEGIN 
DECLARE @StartNumber VARCHAR(16)
DECLARE @EndNumber VARCHAR(16)
DECLARE @Prefix VARCHAR(7)
DECLARE @Count BIGINT
DECLARE @LowVal BIGINT 
DECLARE @HighVal BIGINT
DECLARE @ValueNo VARCHAR(16)
DECLARE @ValReturned VARCHAR(16)
DECLARE @Tab Table

		(
ValueNo VARCHAR(16)
		)

SET @StartNumber = @MinVal
SET @EndNumber =   @MaxVal
SET @Prefix = SUBSTRING(@StartNumber,1, 7)
SET @LowVal  = CONVERT(BIGINT, SUBSTRING(@StartNumber,8,6))
SET @HighVal = CONVERT(BIGINT, SUBSTRING(@EndNumber,8,6))

SET @Count = @LowVal    -- Set the count as the min tissue value

WHILE @Count <= @HighVal  -- While the tissue number is lower than the max parameter value keep looping

BEGIN   
	SET @ValueNo = @Prefix + RIGHT('000000' + CONVERT(VARCHAR(6), @Count),6)       
	INSERT INTO @Tab(ValueNo) VALUES (dbo.ExternalProcessFunc(@ValueNo,1))   -- For every loop insert the values into the temp table
        	SET @Count = @Count + 1 -- Add the value of 1 onto the existing number 

 /*Having problems working out what to do from here onwards*/

	SELECT @ValReturned = ValueNo
	FROM @Tab
	WHERE  ValueNo NOT IN ( SELECT DISTINCT Table1.Column
		       		     FROM   Table1
		                                     WHERE (SUBSTRING(Table1.Column,2,12) >= SUBSTRING(@StartNumber, 2,12) Or @StartNumber = NULL) 
         	                                                     AND   (SUBSTRING(Table1.Column, 2,12) <= SUBSTRING(@EndNumber,2,12) Or @EndNumber = NULL)
    		 IF @ValReturned <> '' 
		BEGIN
		INSERT INTO @MissingVal (MissingVal)
		SELECT ValueNo
		FROM @Tab     
		END
END
RETURN 
END