2013 BMW 3 Series GT issues similar to B+ cable recall

[The Engineer]

2013 3 Series GT (circa 17k miles) auto start stop at red route lights on Friday with wife and 3 year old in car, would not restart when the lights changed. Dashboard clusters intermittently on and off, battery definitely fully charged but warning light saying charge battery. Booster battery turned starter VERY slowly but shorting at the same time. Slight burning smell from left hand side of engine compartment. BMW not interested given its not subject to recall, do people think it's the starter motor?

[Elliot2000]

Why don’t you get it diagnosed- surely something like that would take less than an hour to find the cause and even at a bmw draper you would be talking about £100 for the investigation. And then you maybe will get goodwill from bmw