The sensor in the top of a Corsa water pump is a temp sensor. This signals the ECU. It is the ECU that controls the fan these days on modern stuff. I think the Corsa is like this (I may be wrong)
To check if it is an older system, one would look for a radiator temp switch (which switches the fan directly, like a Mk3 or 4 Escort for example, although Ford is in the thermostat housing) is
If there is a good reading on the temp gauge .. centre .. N, then fan should not be running. If it gets up to 3/4 or more, fan should be running. You should test the fan connector plug with a multimeter to see if a value is changing. Usually, 12v will appear to energise the motor.
If the thermostsat was stuck closed (they usually fail open tho) engine would just rise to overheat, and fan would run constantly. System would pressurise up and throw out water or boil in engine.