The drama unfolded at about 11.30pm on Monday as Mr Owen stopped in Eltham on his way home. He said: "A woman was arguing with the cashier. Suddenly a boy jumped out of the back of the car. He kicked the door of the garage in, ran inside and started thumping the cashier with some force. I ran in and just started shouting at him 'get off '. I then stood between the pair and told the teenager to stay away. I think the boy was so shocked at how loud I was screaming he stopped."