Has anyone else noticed that the interior temperature control in their car is poor?

I have a 170 elegance estate and i find that after a period of time (with the engine warm) set at 22c the car becomes very hot, if changed to 21 the temp plummets. The next long trip I do I will have to take my digi thermometer.

I previously had a passat with dual zone and the control was spot on, perhaps because it had a diffuser vent on top of the dash?

Think it's got something to do with the sensor on the dash - in the middle - think it takes a reading of how hot the sun is coming in through the screen.... right in the middle, there's a we' shiney bit...

Al.