Go to an open area.

Push the compass correction switch, (the small (S) button next to the compass), holding it in for a couple of seconds until the compass display first clears, then starts to flash... let go of the button.

With the steering wheel on full lock, drive the car slowly in a complete circle - this will reset the compass.

If you are unable to find a large enough open area, complete the 360 degree manoeuvre slowly using forward and reverse gears.

If this doesn't work, repeat as above, but turn in the opposite direction.

If this still hasn't worked, it's time to see a mechanic!