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 bag, at the top 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 bag will then load in a box in the middle of the screen.
On the right hand side of this box it says ‘got a voucher code?’ with a drop down arrow.
Click on the arrow and the box for the code will appear.
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.