Relax! Like newms said above, they will be able to match the documents to your name. If you sent your documents via post, they will see your name on the envelope. If you used online application system, it's even easier. At the worst case scenario, suppose that your essays are lost/misplaced/whatever. But as the deadline approaches, the gradsecs will see that you application is missing your SoP. Then they will contact you saying you need to complete your application - this is their job - this is what the gradsecs do. I didn't put my name on any of my SoPs (all online applications), but my applications are complete now. So don't worry. You'll be fine.