They have done that, a diesel nozzle is larger than a petrol one and doesn't normally fit in a petrol car.
Have they changed the nozzle sizes in the past few years? If not, I wonder how Caroline managed to fill her old Citroen up with Diesel, circa 2000 . She was in Green Flag who brought it home for us, but it took me a couple of days to clean out the tank and all the fuel lines, it didn't go to waste though, just threw it into my central heating oil tank.
Pete