Hi Colin the sensor is at the rear of the block under the starter motor.You have to remove the inlet manifold to gain access to replace it although the starter does not have to come off. Its not a bad job really just a lot of work to change a crank sensor.People have reported changing the sensor from underneath without removing manifold but having done loads of these i don't think i could manage it and even so by the time you have struggled to attempt it you could have the manifold off

HTH

Steve