A UNiDAYS code that doesn't work when you apply it in basket usually needs replacing.
UNiDAYS code only applies to full price items, it is not valid for sale/outlet items. This offer cannot be used in conjunction with any other offers, purchases of Gift Cards and selected products.
Go on the UNiDAYS website, log in and generate a new code. Come back to the site, slap it in the discount code section of the checkout and "Voila!", it should all be fine.