The case highlights a growing movement demanding that people who launch unprovoked assaults - so-called one-punch attacks - which result in serious injury or death are jailed for longer sentences than at present.

Speaking to Sky News, Rachel Mason said the three-year, nine-month jail sentence was an "insult".

Paul Mason, 52, died six months after he suffered head injuries outside The Ivy in London's West End in December 2020. He was leaving the restaurant when he was hit and knocked to the ground.

Steven Allan, 34, from Hook in Hampshire was jailed last month after admitting manslaughter. He was cleared of murder after a trial.

During court proceedings Allan claimed he had launched the attack after mistakenly believing Mr Mason had stolen a mobile phone.

Ms Mason described her brother as a "kind, gentle, decent man who had never been involved in any kind of violence".

"It was just shocking," she added, "I just couldn't comprehend it. I couldn't understand how my brother could be dying in hospital. He was a fit and healthy man.

"And you know, somebody's done this to him."

Mr Mason was allowed home but suffered a brain bleed and catastrophic brain damage.

Ms Mason says her older brother, Simon, who was devastated by grief, died three months after Paul. He had relapsed into alcoholism and passed away.

Ms Mason has launched a petition calling for her brother's killer's sentence to be reviewed as well as an overhaul of sentencing guidelines.

"How can someone rehabilitate in a year?" she said. "They can't. We all know that prisons are overstretched and resources are stretched.

"He's not going to get any kind of support in prison in a year. And then he'll be out in the community. It's sickening."

Maxine Thompson-Curl understands that sense of injustice.

Her 19-year-old son Kristian was attacked on a night out for his birthday in Consett, County Durham, in 2011.

He was punched in the head, suffering serious brain injuries, and died 10 months later.

The man who attacked him was jailed for 28 months for grievous bodily harm.

Ms Thompson-Curl, who runs the charity One Punch UK, is backing Ms Mason's petition and campaigning for longer sentences for unprovoked attacks.

"I want to mirror what Australia do, [after] they campaigned long and hard, where they have achieved 15 years for one-punch assaults and that is just for the assault," she said.

"If they die it's 25 years - 25 years, that would be a difference to you, wouldn't it? I've got no idea why, in this country, the most you'll probably get is two-and-a-half years."

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On the other side, however, there is a different view. Jacob Dunne killed a man with one punch over a decade ago. He was jailed for 30 months, serving 14, after admitting manslaughter.

James Hodgkinson, 28, died after spending days in a coma.

Mr Dunne believes longer sentences may not act as deterrents.

"A life for a life, an eye for an eye," he said, "leads you down a very dark and steep slope where nobody changes and everybody thinks they have the moral high ground until they make a mistake.

"And then they're never redeemable again."

Barrister Edward Henry, KC, says the UK sentencing guidelines are very well thought out. While it's an imperfect system, he says it does work.

"Every single life is innately precious. No sentence is going to adequately reflect that," he said.

"But would it not be better that a person who has made a ghastly mistake... and they have recognised that and they have pleaded guilty to manslaughter, would it not be better than to sentence them on the basis that they did not intend to kill?

"That they too will live for the rest of their life knowing that they have killed and that they are deeply remorseful."

Mr Mason's family expect to hear at the end of this month whether they will be allowed to appeal against his killer's sentence.