Jammed Heater Controls On A 2005 Vauxhall Vivaro ?

Hi all , can anyone help me with my problem ? I have a 2005 vivaro and the heater direction control knob will not turn through its full cycle ie it wont direct air to the windscreen only out the dash vents. There is a resistance from the knob when you try to turn it and i don`t want to break it off, its a nightmare in this cold spell as it takes absolute ages to get the cab warmed before it starts to thaw the windscreen, is there a simple fix or could you diy to take the console apart to get into the controls ?

Many, many thanks for any help !!