Well that was pretty painless.
I used the official site to order two Crit'air certificates at the same time.
The Euro 5 Plus 4 with its petrol engine received a Crit'air "1" the best available for an IC driven vehicle.
The Euro 6 diesel Jaguar XE received a Crit'air "2"

Probably fair enough really but it gives an indication of the way the wind is blowing for diesel.
Now all I have to do is stick them on the cars.