I was going to suggest that maybe you had a ladder for a TD5, but then I remembered that the rear doors on the TD5 are the only panel they have in common with the Series I Disco, IIRC. Which is slightly ironic given the problems they had with the hinges sagging. Hopefully the hinges are different but I'm not holding my breath.
You could always upgrade to a nice Patriot roof rack with a built-in ladder :D