Looks like there's a bug on the Zavvi website. If you add a single copy of Halo 3: ODST to your basket, then view the basket and add another, it gives you £10.90 discount.
With Quidco it works out to be £12.05 a copy - great if you've got a friend that wants one too!
Best of all, the discount keeps stacking - Want 10 copies? Fine, and it gives you £54.50 discount.