Hi SteveThis vehicle as two temp sensors fitted.
One for the MEMS / brown in colour.
one for the gauge / black in colour.
as looking at engine N/S of cylinder head look under the housing were the brown sensor is.
the gauge sensor is a 2 pin plug but only one wire in it.
when the gauge goes hot disconnect it and the gauge should drop to cold. if it does the the sensor is faulty.
Also if any work has been done were the cooling system has been drained , it could be air locked.
HTH
DAz...........