Earth rods are fine to use on their own and give a physical return path should your supply neutral become open circuit
Below is the long form explanation:
If an installation has a PME OR TNS EARTH ARRANGEMENT (90% of properties UK) the car must be on a separate earth or have a device that detects the neutral wire becoming open circuit.
If an earth rod is used ON ITS OWN. Exposed gas pipes, water pipes or other items of conductive material (metal) that is attached to the house earth must be more than arms reach from the car under charge.
Must electricians do not like the hassle of fulfilling the above criteria, we have installed over 4000 rods and rarely come across issues.
NEW RULE BS7671 2018 REGULATION AMENDMENTIf an Electronic device is used that detects an open neutral wire and that device requires an earth rod for voltage reference, the earth rod must be over 5 meters away from incoming services (gas and water). This is ONLY the case FOR an Electronic device that still require an earth rod. A LOT OF ELECTRICIANS THINK THAT ALL EARTH RODS SHOULD BE OVER 5 METERS AWAY FROM SERVICES. ITS NOT CORRECT. The reason the rod must be over 5 meters away is incase noise on the line would give the electronic device a false reading.
With the O-PEN device there is no test to know whether the electronics of the O-PEN device is working or not (electronics have a habit of malfunctioning or freezing over a period of time), and if that happened you would not have protection.
An earth rod is a physical product that is always there.