It will be your air re-circulation flap thats stuck open......
Firstly try swicthing your air re-circ button with the heaters on full blast, if you dont hear/feel any difference in air flow then the flap is stuck open
It can be accessed from 2 places, either by removing the driver side scuttle panel then the other little panel that is just clipped on under that you will see a piece of black plastic (It about 8 inches long if that) just slide it up/down (it kinda rotates right round) job done.
It can also be located from the drivers footwell area, on the right hand side just stick your head under and then stick your hands up into the gap, have a fiddle around and u will find it no probs. Yopu will be able to move it to open or closed, just have a play around until you know which way is open and which way is closed. Make sure the blowers are swicthed off when your fiddling about with it though ;-)
Note: Everytime you turn the blowers on and off this will close/open the flap(so even if you dont use the re-circ button the flap is still being opened/closed everytime your swicth the blowers on/off) however mine was sticking half way so i just leave my blowers on all the time so it does not try and open/close the flap. When i get round to it i will have a look at a perm fix for it