heating problems on S1

Hi,

next week i’ve planned my holidays and to ensure I’m not bored to death i have planned to repair the heating on my S1
not that I need it at the moment, but I’m sure the day will come, i’m gonna need it

cuurent problem is that the heater hardly works, ie you will need warm socks to keep your feet warm, the car is equipped with an air con by the way

to ensure I only need to get the front clam off once, i want to replace anypart which could be responsible

so at the moment i’ve got a new heater matrix on the shelf.

any suggestions which other parts could be responsible for the lack of heat