Now, 50 years later and speaking for the first time, the man closest to Sandra, her boyfriend John Hankins, reveals he has been living with guilt over her brutal murder ever since.
John Hankins - who features in the three-part BBC2 documentary Lucan, which airs 9pm tonight - was the last person Sandra spoke to on that fateful night.
The couple should have met that evening but John, a pub manager, was asked to work and persuaded Sandra to swap her shift meaning, tragically, she was at the Belgravia house when the rampaging Lucan arrived.
“Sometimes I feel like I’m the one who actually killed her,” he tells The Sun.
John and Sandra met in 1974, after she signed up with a domestic agency and took a job as a nanny to the children of Lord and L – Frances, ten, George Bingham, seven, and four-year-old Camilla
Sandra had overcome domestic violence, a broken engagement that left her pregnant at 17 and a second child at 21 with a married man. Her parents brought up her eldest son, Stephen, as their own and she gave another, who she named Gary, for adoption.
She went on to marry Roger Rivett but they divorced in 1974 - shortly before she took up her post.
Lucan was a man who lived extravagantly, driving an Aston Martin and racing speed boats.
He was ironically known by his friends at the prestigious Clermont Club in Berkeley Square, founded by his best friend, John Aspinall, as ‘Lucky Lucan’ because of his gambling losses at bridge and backgammon.
Veronica suffered from postnatal depression after the birth of their children and in 1968 Lucan encouraged her to have treatment.
But his increasingly controlling behaviour and gambling addiction led to their break up in 1972 - sparking a bitter battle began for custody of the children.
Living in a nearby property, he would spy on her and record their telephone conversations in which, she said, he tried to provoke her into anger in an attempt to prove that she was of unstable mind.
But Lucan lost the court case, the children remained with their mother and legal expenses along with mounting gambling losses put him ever more seriously into debt.
She was most probably a therapist and marriage guidance counsellor wrapped up in one
Glen Campbell
In local pub the Plumbers Arms, some 300 yards away from the Lucan home, Sandra met John, the area manager for the brewery, and they struck up a relationship.
“We were enjoying life,” says John. “Three or four weeks into the relationship she told me that she had baggage of her own. I learned about her sons and she explained that she had put herself into a facility because she was depressed.”
Sandra, 29, and Veronica, 36, grew close as she found a friendly ear away from the increasingly heated arguments with her husband. The children liked her too.
“I think that when Sandra was in the house Veronica was able to cope with it better,” says John. “Sandra gave her a sense of safety and security.”
BBC investigative journalist Glen Campbell, who features in the documentary, tells us that Sandra was a lifeline for Veronica.
He says: “They immediately struck a bond. They would go out together and enjoy a drink at the Plumbers Arms.
"It was a break for Veronica from her life trapped inside her home where she was receiving ominous, late night, heavy breathing phone calls from Lucan and being spied on by private detectives he'd hired.
“Sandra was fun loving. She could talk to and understand people and get on their level. So, she was more than just a nanny to Veronica.
"She was most probably a therapist and marriage guidance counsellor wrapped up in one.”
John and Sandra’s usual date night was on a Thursday and he would turn up at the Lucan house for her where, on several occasions, he met Lord Lucan, 39, who would arrive to take the children on access visits.
They would often spend the evening at his grace and favour flat but a switch of plan, when the brewery asked John to work at one of his pubs that Thursday, was to have tragic consequences.
Having switched her night off, Sandra put the two youngest children to bed at 8.30pm, then went downstairs to make a cup of tea for Lady Lucan.
But when she did not return, Veronica went to look for her and heard a rustling in the darkness.
A figure charged out of the cloakroom and struck her on the head several times with a piece of lead piping.
As Lady Lucan cried out, her attacker told her to “Shut up” and she recognised the voice as that of her estranged husband.
A fight ensued and ceased only when she grabbed him tightly and painfully between his legs. Having calmed down a little, they then walked upstairs.
According to Lady Lucan’s account, she asked him where Sandra was and he admitted to having killed her.
While he was in the bathroom getting a wet cloth for Veronica’s injuries, she ran out of the house to the Plumbers Arms to raise the alarm.
Lucan fled and was never seen again. Sandra’s body was found by police, stuffed into a US mail bag at the foot of the stairs, having been killed by blows to the head likely from the same lead piping.
The murder was generally thought to be a case of mistaken identity and that his intended victim was Veronica.
But Sandra’s son Neil Berriman - who has been investigating his mother’s murder for decades - thinks this is not the case and Glen believes there is some substance to the theory.
“There is evidence that, potentially, Sandra could have been the intended victim,” explains Glen. “She was causing a lot of trouble behind the scenes as far as Lucan was concerned, being very supportive of Veronica and very vocal against him and his friends. So, she was not just the babysitter.”
Neil, 57, a quietly spoken builder from Hampshire, was adopted at birth and only discovered that his biological mother was Sandra Rivett when he was ten after his mum died of cancer.
He opened an envelope she had kept for him with a newspaper cutting about the Lucan case, along with his adoption papers giving his original name as Gary Hensby.
Sandra’s maiden name was Hensby.
The jaw-dropping discovery set him on a life mission to find Lucan and bring him to justice.
The mystery of the missing Lord has sparked numerous wild theories - including that he shot himself and was fed to tigers at John Aspinall’s zoo or fled abroad on a friend’s yacht.
But in the documentary Neil and John sensationally confront the man they believe to be Lord Lucan just outside Brisbane, Australia.
It follows a tip-off from a man in Perth who said he had met the aristocratic Englishman 22 years ago, posing as a Buddhist monk, who confessed to him that two women in London, “Sandra and Veronica (Lady Lucan),” had karma to be repaid and that he had killed Sandra.
As unlikely as it may seem, John and Neil are instantly convinced that the man in front of them, who has gone by five various names throughout his life, is actually the elusive Richard John Bingham, 7th Earl of Lucan.
“I looked straight at this bloke and I knew right away, that’s Lord Lucan,” says Australian-born John, who now lives in Brisbane and works in law enforcement.
Despite the police dismissing claims the man is the fugitive Earl, he intends to get the Australian authorities to fully investigate him.
Meanwhile, the guilt and anger continues to haunt John Hankins.
“I got her to change her shift so Sandra was there that night when she should have been home with me,” he says.
“And, no matter what people say to me, I can’t forgive myself for something like that.
"There’s nothing I can do about it. It was my fault. If I hadn’t asked her to change her night off… if..if..if..
“Until you’ve been in a situation where someone really close to you has been murdered, you really don’t understand what the feelings of anger, revenge and injustice really mean.
"Sandra was a very good person. It’s important that, somewhere down the track, she gets justice.”
Lucan airs on BBC2 from 9pm tonight and is available on iPlayer
