Disco 1 Heater 300tdi

Hi all. I've looked through the postings for heater problems on 300 tdi and all I have seen relate to it not working. Mine doen't seem to stop working! I've had a look and the mechanics seem to work ok when the heat knob is twiddled- just that hot air blows around the footwells (both sides I think) whatever setting the temp control is on. I'd appreciate any thoughts that might help- other than drive with the windows open which I do!! Ta muchly.