If you have received a gift voucher,  you will need to enter this into the Gift Card box on checkout. 

You will find this just underneath your offer total  - just click on the down arrow next to 'GIFT CARDS' shown below

And a box will appear for you to enter your gift card or voucher number. Once done, the price to pay will be adjusted accordingly and you will see what you need to pay after the voucher deduction. 

Don't worry if this voucher does not subsequently appear on your confirmation, it will have been used as part payment - just check your card statement to set your mind at rest!