If you look at the replacement Richard suggested (mines exactly the same), you'll see that the actual sensor is a small black knob, about the size of a 5 pence piece. It's normally located underneath the unit, but sometimes they are located on the side. It has to be located about 50mm away from any objects (i.e. wall).
If its not a sensor switch, you might find a normal rocker switch, normally covered in some sort of plastic cover.
