The small voice on the end of the phone, cowed by terror, stays with her; the horror of it is still fresh, even now. Late on a summer night in July 2007, Belinda Skudder took a call from her dear friend Joanna Simpson. Clearly she feared for her life.

‘Jo said, “Can you come round?” in a tiny little voice. Three times I asked, “What’s the matter?” She didn’t say.

‘I drove round thinking, “What am I going to find?” When I got there, Jo stood on the front doorstep looking like a rabbit in headlights. She told me Rob (her husband) had threatened her with a knife. He’d stood behind her, held her in a tight grip, put a massive kitchen knife to her throat and told her he was going to kill her.

‘Jo had managed to talk him down. She’d said, “If you kill me, what will happen to the children?” They had a son and daughter, then aged seven and six. He told her, “You’ll be dead and I’ll be in prison.”‘

Three years later these two chilling predictions came true: Joanna Simpson, vibrant, beautiful, talented and much loved by her wide circle of friends, was dead at the age of 46. She was killed in cold blood by her estranged husband Robert Brown, who bludgeoned her to death with a claw-hammer at her home, within earshot of their children Alex, then ten, and Katie, nine.

The late Joanna Simpson pictured right with Hetti Barkworth-Nanton (middle) and Belinda Skudder (left) on New Years Eve in 2007

Weeks before he killed Joanna, Brown, a BA pilot, had dug her grave in remote woodland at Windsor Great Park. Directly under the Heathrow flight path, he buried a plastic garden box which became his wife’s makeshift coffin.

He hit Joanna at least 14 times on the head with the hammer he’d hidden in the children’s school bag — Katie later described the ‘bang, bang, bang’ she heard from her playroom — after he had returned the children from an access visit on Halloween 2010.

Brown had, you may deduce, planned the killing meticulously — but, extraordinarily, a jury acquitted him of murder and he was sentenced to 26 years for manslaughter instead.

Yet in November, in as little as eight weeks’ time — after serving just 13 years in prison — Brown, 59, is due for release.

The prospect appals Joanna’s family and friends, three of whom — Belinda, Amanda Key and Hetti Barkworth-Nanton — have been pleading with the Justice Secretary Alex Chalk to keep Brown behind bars until the end of his sentence.

It seems their message has got through: Chalk namechecked Hetti when he spoke about the case on the BBC’s Sunday With Laura Kuenssberg show this week.

‘The family have my word, and Hetti, her friend, has my word,’ he told Kuenssberg. ‘We will do everything we can to keep her safe and to ensure there is justice in her case.’

Hetti, 56, feels compelled to make this impassioned plea in her friend’s memory to redress the ‘absolute sham’ of Brown’s trial.

A company CEO, she also set up the Joanna Simpson Foundation with Jo’s mother Diana Parkes — run by a board of trustees, now including Jo’s children Alex and Katie — to help the child victims of domestic abuse. Hetti is also chair of Refuge, the UK’s largest single provider of support for women and children escaping domestic violence.

‘Every year an estimated 1.6 million women are experiencing domestic abuse at the hands of dangerous men, and every day Refuge is supporting 7,000 of them,’ says Hetti. ‘So how could I possibly stand by and see Brown, who I believe is a very dangerous psychopath, freed onto our streets without doing anything to stop it?

‘He has been assessed as a “critical public protection case”, which means he needs specialist management and additional resources if he’s let out. And yet a recent report shows the probation service is understaffed, undertrained and overwhelmed. So who will be monitoring him?

‘Jo’s mum Diana wants him to die in prison, to be locked away for everyone’s safety. Alex and Katie fear him. They want nothing to do with him. No woman is safe. I’m frightened — we all are — not least because of my high profile in the campaign to keep him in prison.

‘And the anomaly is that he is in a category C prison and considered too dangerous to go into an open prison, because of the risk to the public — yet he is due to be freed in two months.’

Belinda, 59, adds: ‘I’ve felt nervous that he might get someone to come round and attack me ever since he went to prison. We all made statements about him. He showed no remorse in court. Nothing was his fault.

Evil: Robert Brown bludgeoned his wife, Joanna Simpson, to death

‘He has the traits of a narcissist. He has to win at all costs. If he is allowed out of prison he will feel wronged. No one will be safe. We are dealing with the Devil.’

Amanda, 57, who has two children, the eldest of whom is Jo’s goddaughter, says: ‘None of us could believe it when he was found not guilty of murder.

‘In our view it was clear. He’d come to Jo’s house prepared, with a hammer. He’d dug her grave; lined a plastic box. He portrayed himself as the victim when he was the manipulative, controlling one. It beggars belief.’

Hetti adds: ‘He hoodwinked the jury into believing Jo was a scheming, rich b**** when actually she was the opposite; kind and selfless beyond belief.’

‘There wasn’t anyone who would help and support her friends as she did,’ agrees Amanda, an account manager for a local council, who first met Joanna when they were in their early 20s.

‘She was a great listener and could put everything into context. Over the years she saved me thousands of pounds on therapy.’ She smiles.

It is partly to restore their friend’s reputation, traduced so vilely by Brown in court — as well as to plead for him to remain in prison — that we meet at Belinda’s Berkshire home with its stunning cottage garden.

She lives a few miles from Jo’s former house, Tun Cottage, a glorious mock-Tudor pile in Ascot which she bought before she married Brown.

Belinda and Jo — both passionate horticulturalists — enjoyed going on ‘old lady’ trips to garden centres and the Hampton Court Flower Show together. They shared cuttings and even set up a little business selling potted plants and hanging baskets.

Each of Joanna’s friends has her own special reminiscences. Hetti, married with two daughters, remembers: ‘She was a wonderful hostess, naturally beautiful, late for everything. She loved horse riding, cooking, rally driving; she was an accomplished pianist. And she was one of those perfect mums. Her children were the centre of her world.’

Amanda says: ‘When she put her name to something, she did a superlative job. When she started a B&B at Tun Cottage it was five-star. Her breakfasts won awards.’

That they all adored her is evident. Belinda, who met Joanna when she was pregnant with son Alex in 1999, recalls how she swept into their baby group ‘in a green velvet coat and leggings, her hair really long and blonde’.

Later, spotted by a modelling scout, she won a contract to advertise Activia yoghurt.

She had been married before — to first husband Nigel — whom Amanda remembers fondly. A sense of disbelief persists: how did Jo fall prey to Brown, a man whom Hetti says ‘gets his kicks from power and control over women’?

It was 1998 when his malign presence first cast its shadow over Joanna’s life. She met him while flying to a holiday in South Africa: he was piloting the plane and they were introduced by the purser, a friend of Joanna’s.

The dashing BA captain had the aura of a young Sean Connery. Joanna was smitten. But quickly his controlling, coercive influence prevailed. ‘We didn’t see the underlying control at first, but as soon as the ring was on her finger he changed. He became rude, horrible,’ says Amanda.

A month after they were married, Joanna became pregnant. She resolved to stay with Brown, despite her growing misgivings, to keep their family united.

But her friends noticed the effect he had on her. ‘When he was due home from a long-haul flight, Jo became visibly tense and nervous, though she’d been really happy when he was away,’ says Belinda.

The prospect appals Joanna’s family and friends, three of whom — Belinda (R), Amanda Key (L) and Hetti Barkworth-Nanton (M) — have been pleading with the Justice Secretary Alex Chalk to keep Brown behind bars until the end of his sentence

‘He was really isolating her,’ says Hetti. ‘She was so vivacious, but he would shut her down, undermine her, belittle her.

‘Towards the end he would make snide remarks about her in front of us. She was a wonderful cook and homemaker and would throw brilliant dinner parties that she’d planned to the nth degree. But he would attack all of that. He’d say, “I’m not eating this s***.” He chipped away at her self-esteem.’

‘He sucked the oxygen out of her,’ agrees Belinda. ‘Jo knew the relationship wasn’t right but she wanted to keep it all together for the children’s sake.’ Then, in February 2007, there were sinister intimations of what was to come.

‘He rang from Hong Kong and said he was having dark thoughts about killing Jo and the children with an axe,’ says Hetti. ‘But Jo said, “He’d never hurt me because of the children. He’d never do that to them.”‘

In July of that year came the threat to kill her with the 30cm kitchen knife. Belinda sat with Joanna in her kitchen afterwards, until the early hours when, still terrified, they barricaded themselves in her bedroom.

‘I kept saying, “You must ring the police,”‘ recalls Belinda. ‘But she refused. Later it emerged that she was terrified to do so because Brown had threatened, “It will be the worse for you if you do.” ‘

The next day, however, she resolved to leave him, fleeing with the children to Diana, who lives on the Isle of Man. When she returned home, a security guard moved into the house with her.

Brown, meanwhile, had installed a camera and a car-tracking device so he could monitor Jo’s movements. He would check the burglar alarm log to see when Jo turned it on or off, so he knew when she’d left home. Although obsessively suspicious of her friendships, Jo’s friends believe he had ‘many affairs’.

The acrimony ramped up until, in September 2007, Brown finally left the house.

Although barred from entering it, he used the children to get security codes for the burglar alarm, then copied private documents when no one was in. He stalked Jo in his car, skulked in her garden, jammed his foot in the doorway on access visits.

When her friends recall the black day when they learned he’d killed her, they grapple to describe the enormity of their loss. Joanna died one week before their divorce was due to be finalised.

After he had bludgeoned her to death he ripped out the house phone and the CCTV recorder, which was never found.

Then, with Joanna’s corpse in the boot of his car, he took their children to his pregnant girlfriend’s house. From there he drove to the remote spot in Windsor Great Park where he had already dug her grave — a short distance from a den he had made in which the children had played, quite probably as he dug it — and buried Joanna’s body.

The dreadful news reached Belinda first. An editor with Sky TV — whose own two children are a similar age to Jo’s — she was named as the emergency contact at Katie and Alex’s school. ‘It was Monday morning; I was at work. I got a phone call to say Katie and Alex hadn’t turned up to school; that there were police and cordons around their house.

‘I phoned Jo’s mum. She said, “I think she’s been murdered.” I just sat there in stunned silence. It was so awful. Then I started crying. I drove home and I was howling by then.’

Hetti remembers calling her friend on the night she died. ‘I’d been speaking to Jo loads that weekend. She’d been planning to come and see me. I tried ringing on Halloween evening and her phone was engaged. I left a message: ‘You’re such a gasbag.’ ‘

Justice Secretary Alex Chalk appearing on the BBC 1 current affairs programme

Meanwhile, Belinda was repeatedly calling Hetti at work. ‘Eventually I picked up the phone and she just blurted out: ‘Jo’s missing. Rob has been arrested on suspicion of murder.’ I made her say it again. Then I dived into an empty office and broke down.’

Amanda, then living in Lincolnshire, remembers speaking to Jo the day before she died. Her mood, as her divorce was almost finalised, was buoyant. ‘She said, “It’s all over now. I’ll be able to leave the house. I’ll be able to come and see you.”‘

But then came the news that shattered this hope for ever. ‘I’d got an odd Facebook message (from a member of the Press) asking about Rob and Jo’s relationship. I Googled her name and read: ‘Heiress missing, believed murdered.’ I started screaming.

‘My daughter Eloise came rushing in, saying, “What, what, what?” I said, ‘Something really awful has happened to Auntie Jo.’ I phoned Hetti and it was the worst thing, the kind of thing that never happens to people you know. We were all terrified for her.’

Brown had already admitted to killing Joanna, but it was a few days — November 5 — before he took police to the spot where he had buried her.

Her death had a profound and lasting effect on all of them. For Hetti, the reverberations ran deep. ‘I didn’t work for seven months. I couldn’t keep myself together. I was a leader of people but I couldn’t put one foot in front of the other.

‘I went back to work after the trial thinking I would be fine, but the outcome was devastating. I ended up not working for three-and-a-half years. I was diagnosed with complex PTSD. I also had breast cancer, which I put down to the effect of the trauma.’

Alex and Katie went to live on the Isle of Man with Diana, who told me earlier this year: ‘I scooped the children up and made their lives safe, but of course it’s been my salvation. A total joy. I love them to bits; they’re part of Jo.’

And even now, 13 years on, Joanna is an almost palpable presence in her friends’ lives: in the garden Belinda nurtures; in the dinner service Jo and Hetti shared — ‘She had 16 dinner plates, I had eight and we’d lend them to each other if we had a big party’ — and in her multifarious acts of kindness and love.

‘I remember when I split up with my girls’ father she arrived on my doorstep with a survival package: a bottle of wine, 20 Marlboro Lights (I smoked then) and a Barry Manilow CD,’ Amanda smiles.

They reflect on the fact that she was a ‘perfect mum’. When they assembled at her house after her death, a traybake was still in the oven; she’d made it for her children’s homecoming.

She lives on, too, through the charity that carries her name. ‘We set it up to help children because Jo was such a nurturing mother. Everything was about her children,’ says Hetti.

That Alex, 23 and Katie, 22, have, through their own father’s heinous act of violence, been denied a life with the woman who loved them most, is monstrous. It is why, Jo’s friends all vehemently agree, he must never be released.