[jambo]

A number of issues are possible.

Easiest one to check is that the system's working properly:
1) Check oil and coolant level are OK (coolant at reservoir and that the radiator is full)
2) Start the bike from cold and note the temperature, take it for a very short run (mile or 2) to get some heat in it then just park it up and let it idle. Alternatively just let it idle from cold. The temperature should climb up gradually. When it gets up to 105ish the fan should kick in and bring the temperature down, if the fan doesn't kick in then the bike will start spitting coolant out of the overflow and start flashing a warning about temperature.

The thermister that is used to send the temperature to the clocks is a completely separate unit to the thermostatic switch that is mounted in the radiator and turns the fan on and off. I'd guess one of the 2 is faulty, disconnected, or the wiring to it's gone wrong.

Jambo

Edit: I've not got a temperature gauge on the Curvy, but do on the Kawasaki, so I'm not sure where the temperature sensor is (thermostatic switch is bottom right hand side of the rad as you sit on the bike though). One distinct possibility is that the wiring or the contacts for the temperature sensor is corroded, dirty or damaged giving resistance values that relate to a different teperature to the one that's there.