Originally Posted by RGK20m3

I wonder if the temperature at the rear of the rail is a red herring. Normally, the electric cooling fan control sensor is located either by the thermostat or in the radiator outlet/water pump inlet (which is where Jaguar put the condenser fan sensor). Makes sense as you would only want the fan to run when insufficient air flow through the radiator is indicated by a rise in the radiator outlet temp.
Perhaps the temperature at the rear of the water rail is normally higher and is why your e-fan stays on. And maybe the fan running interferes with airflow through the radiator when normal vehicle movement is causing otherwise sufficient airflow. Just a thought…..
Putting probes in both radiator tanks at the upper hoses will tell the tale. If the temps there seem normal, I would pull the water rails and see if anything is restricted, if not, relocate your fan sensor (or use the OE location at the water pump inlet).