I think it is time to write a very unhappy letter, sent by registered mail with a certificate of delivery, or by courier, to Steve Morris. It is absolute nonsense to say that they can not issue a new CoC. If an error was made, for whatever reason, it is up to them to correct it and, if necessary, advise the authorities that an error was made. This may cost you quite dearly in the future if the French Government decide to place an annual carbon tax on cars.
My feeling is that given the circumstances you would have every right to return the car if they do not issue a CoC with the correct CO2.
They have a legal responsibility to issue a CoC with the correct data. Just imagine the consequences if they issued a CoC for a Roadster but inadvertently put all the data, engine size, wheel sizes, emissions, etc for a Plus 4. This is no different. The data is either correct or it is not, there is no such thing as mostly correct or correct in March but not in May for the same thing.