What age is your C5? There are 3 different systems in use in the C5. I presume being an Exclusive you have dual zone climate control.

On the early C5 with climate control, the common fault is the resistor pack breaking. If you adjust the controls for the fan speed, it may turn on at full speed, and then not at any other speeds. This is a £50 part and 10 minutes labour.

I'm not sure of common climate control faults on later models.