Heater blower resistor. I've changed a couple in the past (different models), usually located behind glove box (small disc shape). Not expensive. That's if a diesel heater set up is the same as a petrol engined car? I presume so. This forum may help:- http://www.vauxhallownersnetwork.co.uk/forumdisplay.php?f=83
Let me know if you get it sorted.
George
August 2007