Looking for advise on an intermittent mini countryman petrol r60 mk1 heater issue.

When the car starts this will normally start the heating motor on full to help clear the windscreen. 

This does not always come on at start up, and intermittently starts with a slower heater speed, which stays on sometimes for 10 to 15 minutes, the full heater speed then kicks in. I am still able to turn off all buttons (aircon, heated windows,etc), press the heater max power up and down and temperature buttons up and down. 

However, I also notice when freezing cold or cold temperatures, this also does not come on. 

Unclear what is the cause. I have had a new air con condenser fitted last summer, had a new heater resistor fitted and then tried a new battery (as had thought the battery was low and was automatically shutting down car functions to conserve energy).

Issue still exists and would like to know how to fix. Any ideas? Could it be a sensor as car has climate control, ecu or any other thoughts?