Try this link to get a new thermistor which is likely to have failed.
http://shop.ukwhitegoods.co.uk/Refrigeration%20Parts/Hotpoint/Hotpoint%20Thermostats/2601739%20Hotpoint%20Fridge%20Freezer%20Fridge%20Thermistor.html
see also:
http://www.ukwhitegoods.co.uk/help/fix-it-yourself/spare-parts/2946-fridge-a-freezer-thermostats.html
Disconnect it from the power socket before working on it.
ChicksFan, September 2011