maybe on some other modells but on a D2 is used by the engine management for additional cooling if the coolant temp exceeds 110*C regardless of the aircon, and that's the case when it will run all the time on overheat fault code which must be erased to stop it(what i thought in my first answer as the OP spoke about a switch which is not the case for the viscous fan)
Cheers sierrafery, I think you are probably correct. I've heard the fan and had a look and seen the other one spinning so assumed it was that, but I probably can't see the air con fan?