If you have a coupon you first need to add any item to your cart, you will automatically be taken to your cart below added you will see;
What would you like to do next?
Enter the coupon code you have - if this is valid it will automatically apply the discount to your order.
You can carry on shopping and adding more items to your cart and the discount will change accordingly. Your total cost will update to ensure your whole order is discounted by the voucher amount.
So if your first item added is £10 - add the coupon code (in this instance a 10% off coupon) and you will receive a £1 discount and this will be shown on your cart.
If you then add another item of a £10 value your cart will update showing a £2 discount - you only need to enter the coupon code once. Your discount will be continually tracked.
Please note coupon codes are only applied to products and not shipping costs