I had the same problem 2 years ago, so phoned up hotpoint they sent an engineer round who charged me £89.98 to change the evaporator-thermistor (The £89.98 didn't include the part, this was purely 5 mins labour & tax, turns out the actual part would of only cost me £8.62 in 2004 if it hadn't been under 5yr parts warranty). Two years later I'm getting the same problem, cold freezer, warm fridge!! if I was you I'd try looking on http://www.espares.co.uk for a evaporator-thermistor that fits your machine as this controls the temp in the fridge. Mine's on order, so now all I need to do, is work out how to install it myself!!!
James
December 2006