How can I fix thefridge on my Hotpoint Iced Diamond Fridge Freezer ?

Bought this fride freezer second hand about 6 months ago. Has worked fine until last few days when fridge started playing up. Regardless of what level the dial is set at (even at 1), half the stuff in the fridge is frozen solid, particulaly the stuff in the door compartments.

It seems as though the fridge is not registering when it's reached the appropriate temperature and so keeps just keeps getting colder. Is this a thermostat problem, and if so,what can I do about it? We really don't have the money to buy a new fridge (even second hand) so I'd like to try and fix it myself if I can get some help. By the way, the freezer part of the unit is still working fine.