I've just had the same problem and think I've solved it by
a) removing the grill at the bottom (pull it by the top corners until it releases) and giving it a clean. Mine had a 1cm thick layer of fluff preventing air circulation!). If you look with a torch past the cooling grid you should be able to see the fan moving.
b) putting a 'sticky fixer' or similar on the top of the fridge door where the light switch goes. This is just to ensure that the switch operates properly - if it does not then neither does the circulation fan). The door drops a little with age and this is a simple way to deal with the switch contact. In a quiet room with the fridge door open, manually push up the door switch; as the light goes out you should be able to hear the fan. Keep the vent at the top back right hand side clear also.
Quarter-p
November 2007