Rad fan constantly on but only when driving?
 

just wondering if anyone can help me solve a problem with my 1996 uk wagon turbo. At idle it warms up fine, hot air from heater, all hoses get hot, fan kicks in and goes off again, no leaks, no coolant loss or spill, temp gauge stays bang in the middle. When I'm driving though, when temp is up, I hear the fan kick in but it never goes off, 25 mins later still on when I get to work or home. I recently changed the coolant sensor thinking it might be that but fan still is constantly on once it kicks in after getting up to temp, and not long ago had new water pump and rad. any ideas? maybe faulty thermostat? my header tank cap is 0.9, is this correct? as said, no leaks, no overheating. its driving me mad! cheers and appreciate any advice you have

Any ideas anyone?