If you have used a voucher when placing your order, please don't worry if it doesn't appear as a discount on your confirmation.  Vouchers are used as part-payment, so will be deducted prior to any payment being claimed. You can set your mind at rest by double-checking your form of payment, and this should be equal to the value of the order after the voucher has been deducted. 

We also mention this on the confirmation, just under the total value of the order.