I can almost guarantee that the sign said that it was on the 18 packs, somewhere. Anyway, the point is the manager didn't do anything wrong by not offering you the same deal. There's a reasonable suspicion that he wasn't actually allowed to do so. You had the option to return your beer for a full refund, or you could take the beer at the price you paid. You decided on the latter. Just because customer services then decided to give you a goodwill offer does not admit guilt. You got £100 out of it, I'm not sure why you're complaining or why you think you know better than a store manager.