Well they deal with applications on a rolling basis instead of gathering them in for one deadline, so there is still a chance that you'll get in - your undergrad degree is certainly relevant. I did my undergraduate degree in International Relations as well.
If I were you I would write a personal statement to highlight how your curriculum could map onto the IPT program, and what you hope to take away from it. Whatever you do, DON'T say anything like "I want to work in government/NGO/UN after graduation" - the guys in charge of the program really hate it because it's generic, unimaginative, and doesn't really correspond to the content of the course (IPT is not IR theory).
If you want, I can send you some more info on the program?