The heater control resistors have gone. Remove the glovebox, to the left there is a raised cover, remove the 2 screws. Carefull they will be hot if you have had the fan blowing on speed 4.  Lift out the unit, and you will see the 3 coils, one will be broken.  Take this to a salvage yard to replace, try www.roverbitz.com I get my bits from them.
www.garyianward.co.uk
                
 Gary Ian Ward
                
 March 2008