Officers from the Metropolitan Police Service’s Roads and Transport Policing Command have released a CCTV still taken on-board the route 257 service.
It was travelling along The Grove on Saturday, August 1 when the incident is believed to have taken place, at around 12.30am.
The suspect has been described by police as a bald white man, around 50 years old.
The two images show him wearing a blue and red zip up jacket worn open to reveal a white T-shirt with writing across the front.
Anyone with any information should contact PC Andrew Morland on 07775 030912 or Crimestoppers anonymously on 0800 555 111, quoting reference 187644.