To update on my previous reply. I was out in my wife's 108 TOP last night. Drove about 80 miles, outside temperature was 5 degrees. After approximately 3 miles the temperature in the car was roasting hot. Had to turn the heater down to about 75% level, or open a window slightly. The heater in this car is better than any other car in our family. We have four Vauxhall vehicles, all are inferior to this in terms of heat output. My Mokka has dual zone climate control, takes forever to get up to a decent temperature, and never gets as hot as the 108.
So I reckon the OP has a problem, and needs the car thoroughly checked.