I just finished a postgrad there, so hope I can be of help. Which programme are you applying for? They have ten from what I can remember. The writing sample I gave them was an essay from my last year of undergrad and had nothing to do whatsoever with my SOP. It was a critical commentary on some postmodernist plays.
Oh, and I'm an international student too from a thoroughly non-English speaking country, and I got a first for my master's, so I wouldn't worry too much about the writing sample not being up to par. You might want to have someone look over it for you, though, if this is your first time writing.