Whether you have signed up through a refer a friend link, or received referral credit from your friends purchase, you can use your referral credit in the same way.
Once your cart value is £40 or higher, you will see an additional box appear under the 'gift voucher code' section (see below). This is where you can see your available balance and the amount of referral credit you currently have.
In the box you simply type the amount you wish to use on this order. Note you can use any amount you want within your referral credit limit. Click 'Apply' and the amount you have specified will be taken off the total amount of your order.