How can I repair my Thermor Electric Water Heater GB 75 N?

My 3 year old Thermor (Made in France) 2200W 230V~ Electric Water Heater made a slight hissing noise for about a week and the water was scalding hot, but then the noise stopped and the water became tepid. Is the heater 'cal'd' up and needs descaling? or Does the thermostat need replacing? or Will I have to replace the whole unit? Thermor state a life expectancy of up to 20yrs!