As darkstar points out it depends on the type of heater/aircon controls you have. The flap you refer to is coated in foam in order to make an air tight seal as far as I can see.
If you have electronic heater controls then you could try opening the bonnet, turning on the ignition and asking sombody to watch what happens when you press the top left button (marked with a recirculation symbol) on the heater control panel.
I think you will find the spindle moves to a different position. It does on my van anyway and this is what darkstar is suggesting too. Obviously if it doesn't move there could be a fault.
There was no filter for air coming into the front air intake on my bongo. A failing given the amount of insects and fluff I had to remove to get the aircon to work! I have some photos of a simple fix TBA.
