Welcome to Just Answer
The engine in your car is the 'k' series engine, they are proned to head gasket failure, but what you are describing dosent sound like this, as the temperature guage has not risen too high, and I'm assuming you are not using water or having to top up regularly.
I have pasted below a generic guide that I have written for the cooling system, if you follow it, it will help you to diagnose the problem.
It is likely to be th engine coolant temperature sensor, there is no switch on your car, the fan is ECU controlled, and takes its reading from the sensor.
Hope this helps, if you need more info, such as the cooling wiring duagram, please let me know.
You need to start with the basics, check the coolant level, if it is low top it up and look for any signs of a leak, if it was low and you cannot see any leaks, then get a block test done on the car at a garage, it will cost around £10 and will give an indication as to head gasket failure.
Over pressurising may also be a sign of head or head gasket failure.
Next I will describe the operation from cold of the cooling system, any point that does not happen will indicate a fault with that part of the car, so follow the guidelines below and it will lead you to the cause.
If possible use a laser thermometer, otherwise be careful when testing for heat or temperature.
With the coolant at its maximum level, and the bonnet open, start the engine.
Turn the heater on to hot, and the blower on low
As the engine runs, the WATER PUMP circulates the coolant around the engine, test this by ensuring that the heater pipes start to get warm and blows warm air into the cabin, also the engine should get warm evenly and have no hot spots.
As the car warms up, the TEMPERATURE GUAGE in the car receives different resistance signals from the TEMPERATURE SENSOR and the gauge will rise.
As the engine reaches between 80-95 degrees C, the THERMOSTAT should start to open, and the top radiator hose should start to warm up, and the radiator will in turn start to warm up from the top downwards.
The warm water should flow steadily through the RADIATOR and out through the bottom hose indicating that the radiator is free from blockages.
As the coolant in the radiator reaches between 90-98 degrees C, the FAN SWITCH closes the circuit and sends battery voltage to the RADIATOR COOLING FAN, which in turn starts up and cools the coolant in the radiator.
For Cars with a viscous fan, usually rear wheel drive cars and vans, the fan should become stiffer to turn as the engine heats up
When the engine and coolant are hot, the coolant becomes pressurised, this stops the water from boiling and causing air locks, the pressure is controlled by a valve incorporated in the CAP of the cooling system and the correct operating pressure will be marked on the cap in bar, the cap can be tested with a special tool.
By reading this, and following it as the engine is running, you will understand how the cooling system works, and anything that doesn't happen as it should will lead you to a potential fault.