The man seen on CCTV is described as dark skinned, with a full beard, wearing a light baggy top with a satchel across his chest.
Detective Chief Inspector Sheila Stewart said: "It is vital we track down this man who was in the precise area at the time Mariana was attacked and killed."
More top news
Anti-fox hunting campaigners marched from on Downing Street to oppose controversial promise to hold a vote to repeal the Hunting Act.
Thousands of drivers were caught illegally using their mobile phones on London's roads following a major crackdown.
A motorcyclist is fighting for his life in hospital after a crash with a car in Kensington.