Mr Manning had worked hard to find any relatives, but until the call his efforts had drawn a blank.
"They were thankful for what we've done. [The estrangement] was through no fault of anybody's really, it's just how things have occurred for the family.
"They tried to make contact over the years but it wasn't reciprocated in some sort of way."
Mr Manning said Mr Grant's known relatives lived in New Zealand, although outside of Wellington.
They didn't attend yesterday's funeral but were pleased that 200 to 250 people had shown up to The Metropolitan Cathedral of the Sacred Heart, where it was held alongside Sunday mass.
"I think they were relieved and surprised. They weren't really aware that we could do something like this.
"[They were] grateful he was looked after and his funeral was done with some dignity and respect."
Mr Grant grew up in Stratford and spent some time in an orphanage after his parents died when he was aged 9 and 12, respectively.
He then moved to Wellington and worked for the post office for 50 years. His few friends knew him from the days of the Wellington Workingmen's Club.
Mr Grant's body was found in his inner-city apartment, near Parliament, earlier this month and it's believed he lay there for up to two months.
Among the few possessions found at Mr Grant's apartment were a Bible and his cat's ashes. These were placed in his casket and will be buried with him in Stratford.
Mr Manning said he'd invited the new-found relatives to attend the internment and they said they would think about it.
Today, Mr Grant's cousin recalled him as "quiet".
"I remember Jim and I, we were a lot like chalk and cheese. We had virtually nothing in common at all," she told Fairfax.