Guinness, it could be because the diesel is a heavier block and takes longer to warm up, it is taking longer than you are used to.
As already said, the CC tends to trickle out air until it gets to whatever temp. is deemed warm enough and then it chucks it out at a fair old rate.
I was used to a more erm....manual system in my old car and initially found this strange. Now though I am used to it and generally leave it alone. Only if the window is steaming up or whatever do I adjust the airflow (or if I need to warm my feet on the way home from a night ride @ Thetford!!)