I think the warning in the instructions is no more than covering their backs if something goes wrong. Any electrical appliance can catch fire but we all take the risk that they won't and in the end it's the insurance who take the risk. It does raise the question though, which an insurance company would be entitled to ask in the event of a loss as to why the charger was left unattended when the manufacturer advises against it.
I've used the 12v socket method for years on various cars without a problem, but both Morgans have a direct battery umbilical.
