I'm unsure of the exact location on your model, but it is often just to the left of the keypad. If you run a paper clip along the top of the screen surround you will locate the actuating magnet and the switch itself will be in the corresponding position when the lid is closed.
Hi thanks for such a quick reply, I was looking into how to switch off the screen without switching off anything else. It indicates on this page http://www.sony.co.uk/product/vn-c-series/tab/editorialarticle1 under the 'lifestyle' tab under the 'backlit keyboard' heading that this is possible. I am assuming you are meaning the mechanism when I close the lid.