Danny Burkhill, whose grandfather John is famously known as the 'Mad Man With The Pram', told how he was walking home in the early hours of Wednesday morning through the city centre when he was approached by a man and woman, both armed with knives.
He says the man held a large hunting knife to his throat and ordered him to hand over his phone, which he did.
Still traumatised by what had happened, Mr Burkhill spoke to a homeless man on The Moor, whom he knows only as Neil. He says Neil joined him to look for the attackers and they were subsequently aided in their search by around half a dozen other homeless people.
"I would like to thank those homeless people for trying to help me, and I want to find them as they also helped me with giving evidence to the police," said the 25-year-old cleaner from Woodhouse.
"It's fantastic what they did, thinking about somebody else before themselves. They're like that with my grandad. If they have any spare change they'll put it in his bucket."
Mr Burkhill said he was originally approached at about 5.15am in the Peace Gardens by the woman, who he says kept repeating the word 'pound'.
He eventually gave her a pound and walked off round the back of John Lewis, at which point he says a man appeared and held a knife to his throat.
"The knife was cutting into my throat and the woman reappeared and started speaking normally, asking 'where's your phone?'," he said.
"I could feel the knife going deeper and deeper and I thought if he moves that's it, so I said 'it's in my pocket'.
"They took my phone and the guy said 'you're lucky we haven't killed you', before adding 'you haven't seen us'. I just ran and ran."
Mr Burkhill described the male attacker as black, around 6ft2 and in his late 20s. He said he was wearing a black fleece with a hood that covered most of his face.
He said the woman is about 5ft4, of mixed race, and has black hair. He added that she was wearing a black jacket, white leggings and a white top.
Danny's grandfather John has raised hundreds of thousands of pounds for Macmillan Cancer Support by collecting donations as he pushes a trolley around the streets wearing a green wig and shorts. The 78-year-old recently completed his 25th Great North Run.
Anyone with information is asked to call police on 101.