Thanks for the support everyone.
DS was on the bus coming home from school. It was quite full when he got on, an ok'd woman was sat by the window in the first row of "priority" seats behind the driver, DS sat on the other side, on the aisle seat. He was twisted round talking to a friend. Another lady got on, middle aged, the old woman heaved herself up, leaned across the school girl next to her, through 2 people standing in the aisle to whack DS on the top of his arm telling him to get up to give the middle aged lady his seat.
He moved, but nearer the end of his journey, after talking to his friend, he walked back up the bus and (using DS's word) "discretely" took her picture.
The bus does have CCTV, the police have viewed it. They can't see her hit him, but the rest of the scenario, as he describes it can be seen. He says he wasn't hurt, but wax shocked.
What made me most proud was that he said if it goes to court he doesn't want to give video evidence, he doesn't want to be screened, he wants people to see he is not afraid to tell the truth.
To be honest, we have been surprised the police took it so seriously.