Having recently experinced a similar problem on a 33
the easiest method to check the sensor switch (located on the base of the radiator)
Is to pull the connectors from the sensor and touch together.
The fan should now work regardless of the ignition switch
IF not Then the fan or electric supply is faulty
IF so replace the sensor
These readily available as pattern part from Motor Factors.
Mine cost about 5GBP
Many modify the electric to include switch and relay to override the
Sensor manually from