The voucher code needs to be entered in the Shopping Bag before you go through to the Checkout.

To do this; 

Hover over the white Shopping Bag, at the top right of the screen.
It will drop down the products in your basket.
Below this list will be a ‘View Shopping Bag’ button – click on this.
The 'Enter Promotional Code' box sits to the right of the Shopping Bag Contents
Once you have entered the code; click on the ‘Apply’ button.
The price will update to reflect the discount.

You can now proceed through to the checkout.