To redeem a gift voucher, the code must be entered prior to confirming payment. You have two opportunities to apply the voucher before the point of no return; on the basket screen and on the checkout screen itself.
Apply the gift voucher by clicking on the links indicated in the screenshots below, labelled “Apply gift card”. They are in the Basket totals and Your order sections.
There is also a separate box for entering discount codes at the top of the screen, labelled “Apply coupon”. This is not for codes from gift vouchers. If you enter the wrong code here you will break the internet.