Problem solved, mods please lock if you see this, thanks 
I started the game tonight and it made me download a 38Mb download (not sure if this is new, I haven’t played for a little while).
Then the Marketplace star appeared, so I had a look but couldn’t see any new marketplace content. No big issue there.
I went to dashboard and loaded the game again. After selecting the Hard Drive as my storage device it then gave me an error message ‘Transfer Failed: The Halo 4 server is not available at this time. Please try again later’.
I clicked OK and it then said ‘Unable to validate: Please make sure you are connected to Xbox Live and have the War Game Pass downloaded’.
I went to the main menu after this but thought I’d back out to the dashboard again and reload as I’ve found that corrects problems more often than not.
Well, the same two error messages appeared again. This time I proceeded to the Inifinity menu and it’s telling me I don’t have the Castle DLC, when I do.
Has anyone else experienced this problem or had similar? If so, how did you solve it, did you just redownload the Castle DLC?