[mike1967]

My son has a 2008 mini one petrol, his fan has started to come on immediately as soon as he starts the engine and stays on for the entire journey and turns itself off after engine being turned off for about 5 minutes, the coolant level is fine, can't see any leaks anywhere and no warning lights have come on, any ideas of what could be causing it?

coolant sensor/ or sensor wiring, or ecu issue at worst,
the two wires to sensor one should have around 3.2 volts unplugged, 0.5v around 90c 5v ish at -48c, the other is a switched via ecu earth return,
below pic open it in picture view on laptop and car enlarge it shows the ohms and temperatures you should see on sensor

 

[mike1967]

added point i believe there is a recall on some cars for temp sensor and thermostat housing , worth phoning mini main stealers and ask