I havn’t tried a diode or resistor but I can do that if you let me know appropritate values.
I should have added in my initial report that the vehicle(s) do notice that something is in the chage port and that it is a charger. If I put the bare cable into the car with the other end disconnected I get a different error in the vehicle to when the cable is in the charger. In this case the vehicle says “follow instructions on charge point”