I got an e-mail back from them:
Hi
Thank you for your recent order, and very sorry to hear you have received
the wrong disc - I can fully understand how disappointing this must be for
you.
I will ensure that the correct item is dispatched to you immediately, and
that a FREEPOST envelope is sent by first-class post today to enable you to
return the wrong one to us.
We would be very grateful if you could enclose a brief note with the
returned item, clearly stating the order number (if possible), along with
your name and email address and the reason for return.
I very much appreciate your understanding and co-operation in this matter
and I sincerely hope that the inconvenience caused to you on this occasion
will not deter you from using our service again in the future
Kind Regards
Callum
CD WOW! Customer Services

Hopefully I'll have the right DVD soon.