if you have the heater in the screen position and its not heating then simply follow the black pipes from the sides of the heaters up behind your dash, they sometimes have a tendancy of coming loose in my experience and just heating up your dash!
Cable tie or a few self tappers will stop that from ever happening
or sealing all the joints with duct tape
Because lets face it, with a Mini you should always carry around some duct tape and a screwdriver! if it cant be fixed with those two tools, you've really broken something! haha