Police divers are to search a pond on Hampstead Heath for the body of a man who went missing eight years ago.
Mr Duff, who would now be aged 45, has not been seen or heard from since 12 January 2013 when he failed to attend a family celebration for his daughter's 18th birthday.
Police say this was completely out of character for the father, from Kilburn, north London, and his family subsequently raised the alarm.
Officers attended his flat and forced entry but there was no sign of him. He was formally reported missing the following day after he failed to return.
In 2018, five years after his disappearance, police launched a murder investigation based on information brought to light during one of many appeals.
On 14 May 2018, two men, aged 70 and 51, were arrested on suspicion of murder in Archway, north London.
They were taken to a north London police station for questioning before being released under investigation.
Detective Inspector Tom Williams, of the Metropolitan Police’s Specialist Crime Command, said his team “remains committed to finding Robert and providing the answers that his family are so desperately longing for”.
He added: “Following previous appeals, information was provided to my team that has led us to conduct a search of the No1 pond in Highgate. This work will continue over the coming weeks.
“Understandably, while this search is ongoing, this will be an incredibly distressing time for Robert’s family. I would ask anyone who has information about where Robert is, or what may have happened to him, to contact police.
“Not knowing what has happened to a loved one is a torment no family should endure; if you can help, please do the right thing and contact police and give Robert’s family the closure they deserve.”
Anyone with information that could assist police is asked to call the incident room on 0208 358 0200 or dial 101.
If you do not want to speak to police, you can contact the independent charity Crimestoppers anonymously on 0800 555 111 or online at https://crimestoppers-uk.org/