Yorrick
I have exactly the same issue with my GDI reading 10% too slow. I tried to reset using the web based instructions for the speedometer manufacturer but that didn't work. I contacted the dealer and they don't know how to reset the speedometer so it has been referred on to Morgan. That was 5 weeks ago and I haven't heard anything yet from the factory.
If the speedometer over states the speed by up to 10% that is acceptable but to understate the speed is not and puts us at risk of speeding fines. If/when I get an answer from Morgan I will post on the forum but don't be fobbed off by Morgan. If anyone else has the same problem please let us know so we can have evidence that it's a common problem and they need to take action to sort it, mine is a mph speedometer not kph but the error is identical. Which dealer are you using?
On a more positive note my car is virtually watertight so perhaps some adjustment to the hood may resolve that particular problem with your car.
