I found these instructions in the operator manual
Recalibration
If, after a day or two in operation the setting scale does
not correspond to the water temperature, follow the
following procedure to recalibrate the heater:
Switch off the heater and unplug it from the mains
supply
Measure the water temperature with an accurate
thermometer
Turn indicator arrow (red) to
point to the measured temperature
Set the desired
tank temperature with the temperature selector ring
(blue) .
Hope this helps. Please be sure to use an accurate thermometer when recalibrating the heater.
Dee