I am sure you are right. The only advantage I could see in BMW product and connecting to CAN-bus is the inference in the manual that one of the CPUs on the CAN-bus can control the charger (see attached photo extract). Whether that is and advantage- who knows 😀
Thanks for the response, so if I crocodile clip a good trickle charger to any permanent live&neutral (eg the connectors for external battery jump start) it will work, or get the can-bus charger either will work?