I recently had a similar issue. I received "The Office (UK) Complete Series" on DVD (which is a Warner title) as a gift and discovered after a couple of months that some of the discs were defective (no visible scratches, but they froze up at the same place on every DVD player I have).
I followed the following instructions on the Warner Home Video website FAQ and received a replacement about a month after I sent in the defective set:
17. Can I send a defective DVD I purchased at a retail outlet to you if the outlet will not accept it as a return?
Please return the product to us so that our technical department can review it. Please note that the DVD must be returned in its original case. If you are returning a box set, you must also send the whole box set back. Please return to the following address: Warner Home Video, 4000 Warner Blvd., Burbank, CA 91522: Attn: Business Affairs. Once the product is received and reviewed someone will be in touch. Please remember to include your contact information.