Thanks to Cadman for the heads up.
Activision are sneakily letting people download the Nuketown bonus map after retailers who offered it with preorder copies objected to them bundling it with the DLC update. Here's what to do....
Go to https://support.activision.com/nuketown
Log in, make sure your Xbox is linked to your account and then fill in the details.
Select my preorder game didn't come with a nuketown code, and bingo..... They then send you a code to redeem