Early Interceptors have the screw-in style fan thermo-switch.  We feel these switches are far superior to the later style used on the Mk3. This switch comes on at about 82 degrees C or 180 degrees F which is the same temp that your thermostat should open.  This switch shuts off at 77 degrees C or about 171 degrees F.  The switch might look a litle different than in the picture.