The deal is, if you spend £30 or more, you'll receive £5 off your order, so if you order the newly released Def Jam Icon for £26.99 and then order something along with it for £3.01 or more, you'll receive £5 off the total offer.
This will effectively give you the £3 item for free and give you a further £2 off the game.
Def Jam Icon (XBOX 360) is currently £26.99 and the next cheapest price is £30.99 after delivery but it's currently out of stock, so the price after that is £35.49, so that's a pretty big saving before you've even had a chance to use the voucher.
Burnout Dominator (PSP) £17.99 (next best price is £24.32 from Canada)
Make sure to check the "when bought with" offers as well, as they sometimes have worthwhile items and checkout some of the other good deals they have going on as well, which have been recently reduced.
Don't forget to go through Quidco to receive 9% cashback and 2.5% with the reward card as well, effectively giving you a 11.5% cashback.