mines around 4c too high i reckon, not sure when this happened as it used to be fairly accurate with weather reports and my own temp sensor in the house.
You can adjust it + and - 3 degrees. Its a similar procedure to resetting the amber maintenance light. Instead of pushing both SELECT and RESET buttons before you turn the key on (to reset the light), turn the key on and then push and hold both bottons. You can then manipulate the temp display plus and minus 3 degrees.