And whilst we're on the subject, anyone returning Hardware to a shop in the UK, do not accept a repair if you are still within warranty. Getting goods repaired will invalidate any right to get it replaced if the repaired unit fails, you would only be able to get it repaired again, even if it's still under warranty. Always demand a replacement if it is within your rights to do so.
if anything is dead when you buy it(within a week or two), the store is obliged to replace it on site, if else, they have to send it to the manufactuer's repair center for a warrenty repair.
on the subject on the game, if it isn't in the box, the game store is obliged to either find the disc, that you need, or replace it with a proper one, pirate, nothing, it's just the store jerking you around.
make sure you keep your reciept.