The Td5 crank sensor is one of the few things that can leave you stranded on the side of the road. If the Td5 ECU does not see sufficient RPM it will not fire the injectors and it will never start. A good quality crankshaft sensor is a must, as is keeping spare in the vehicle.
The crankshaft position sensor (CPS) picks up a signal off the flywheel and relays it to the ECU. Over time the tips degrade, especially if oil and dirty is able to enter the bellhousing.
Located on the top drivers side of the bellhousing these take minutes to fit, secured with one bolt and one plug. Easiest access on Defender Td5 is from above (due to the amount of room at the back of the engine) and normally from below on Discovery 2 Td5. The wiring to the Td5 crank sensor should also be checked as it is common to become damaged, especially during a clutch change. If damaged please see the crank sensor overlay loom. Pay careful attention to whether or not a spacer is fitted to the crank sensor, if it is then it must be used to prevent contact with the flywheel. Crank sensor spacers are available here.