Going slightly off-topic, you may also want to rewrite your function as an itvf and use it with CROSS APPLY in your FROM clause instead of using the scalar function in the SELECT list.

CREATE FUNCTION [dbo].VAT2 (

@Net_Amount DECIMAL(25,13),

@Order_Date [date] )

RETURNS TABLE WITH SCHEMABINDING

AS

RETURN(

SELECT @Net_Amount *

CAST(CASE

WHEN @Order_Date >= '2011-01-04' then 0.200

WHEN @Order_Date >= '2010-01-01' then 0.175

WHEN @Order_Date >= '2008-12-01' then 0.150

WHEN @Order_Date >= '1991-01-01' then 0.175

WHEN @Order_Date >= '1979-06-18' then 0.150

WHEN @Order_Date >= '1974-07-01' then 0.080

WHEN @Order_Date >= '1973-04-01' then 0.100

ELSE 0.000

END as DECIMAL(25,13)) AS VatAmt)

GO