Where is the coolant level sensor located on a S type
Jaguar?

 

Looking toward the front of the car inside the engine bay, the level sensor is to the left and downwards from the hose connection to the expansion tank. It is easily removable unless badly gunked in. However I don't think you can buy one seperately to the expansion tank assembly.

​​​​​​Do a forum search and there is plenty of recent info on the level sensor and expansion tank.