Vito interior heater issue
 

Hi,
I have a 2003 Mercedes Vito (Diesel) 108 CDI van and the heater in the van can take nearly 10 minutes before any heat starts to come through. This is from a cold start to driving straight away. During winter it was horrible as you can imagine. The heater booster button works but makes no difference in those 10 minutes. Can anyone advise me what may be wrong or what I could look at? I would have to take the van to a garage as I prefer someone with experience to work on my van, though it would be good to have some knowledge of whats possibly wrong.
Cheers
Lee