How do I account for debt when I calculate Zakat?

I have read this question but it did not help me. I have two bank accounts. One for savings (wherein I periodically pay off any interest incurred), and another for my current money. I have an **interest free** overdraft that I am in, and have been in since last ramadan. My savings have increased by £400 since last year. In this situation, my savings are out weighed by the debt of my overdraft. But my overdraft is interest free so I am not paying money on it. Do I still need to consider it for Zakat calculation? Do have the nisab amount?