zafira diesel running cool

hi there, i own a 2000 'w' reg zafira diesel that takes ages to warm up. i can drive it 4 miles and leave it idle for 10 mins and the temp gauge is still v.low...if i drive it hard or rev it alot the needle will rise up to the correct temp and stay there... it seems strange as there is always hot air in the car no matter what the gauge says... vaux garage say this is normal as the diesels take a while to get warm..IS THIS CORRECT?

thanks for any help:grinyes: