I have exactly the same problem with my FF93 fridge/freezer. Although I have not yet tried replacing the thermistors, I have managed to find them.
If you remove the drawers from the freezer, the inner rear panel can then be removed by undoing the 4 fixing screws (2 top, 2 bottom). This will reveal the freezer (green) thermistor in the centre just below the fan.
Loosening the 2 screws (top left & top right) will then allow the fan assembly to be eased forward permitting the removal of the polystyrene panel. The steel panel underneath can then also be eased forward (careful, as there are cooling(?) coils fixed to this near the bottom!!). This should enable you to see the evaporator (yellow) thermistor, clipped to one of the main cooling coils, on the right hand side.
As I said, I haven't replaced my thermistors yet, as, until I discovered this site, I didn't know that was the solution to what appears to be a common problem. I would be interested to know how you get on, any problems encountered, and if in fact this cures the fault.
Gordon
Gordon, December 2005