C200 heater blower is weak

Although all the speeds change the airflow of the blower on the heater of my 2000 C200 sport estate is weak. I first noticed in the summer when I went to get the air con checked and it was fine but the guy said the lack of performance from the blower was making it feel poor. It's the same now I'm using the heater. Can any one advise me please? I was wondeing if there is a pollen filter or similar that might be clogged and causing the restriction of airflow? The car has done 112k and is regularly serviced. Thanks Ed