Just completed replacement of this, with the kind on-line help of webserve.
In a nutshell, this component can apparently be responsible for sudden cutting out of the engine. The component can't be tested by measuring resistance or anything - just has to be replaced if suspected.
To replace, remove radiator top hose (when car is cold, and pressure cap released). Inboard removed easier, swing up hose if outboard cannot be removed easily. Do *not* try to remove next hose down as this is for the hydraulic system. Sensor is external to the crakcase, to the right of centre, and is secured with a single allen key (which may be tight). The wire should have been secured to a stand-off bracket with a cable tie, and is terminated in an easy pull-off connector, once you have snipped the cable tie round that. Replacement is a simple reversal of all the above. After re-securing the radiator hose worm clips, refill the header tank, but leave it open until running the engine for a moment removes the rest of the air in the system and allows you to fill up again.
Allow a couple of hours. The actual replacement of the sensor takes 5 minutes, but the other parts to the job, and snags, can extend the time somewhat..
Cheers all Alan