Replace Thermostat on Fridge Freezer APM 6842?

I got a guy round to do this on my APM6842. He just pulled the cover off around the thermostat control and then I think it came away pretty easily. He just fed a new thermostat through the revelaed hole into the side of the fridge and put the thermostat in place.
However, this hasn't cured my problem - it's still not cold enough. So I think the problem may be the defrost sensors as others have suggested. Or maybe it's because the flow between the freezer and top isn't working well - apparently the fridge is cooled from the freezer section.

Al
September 2008