Once again this week the laconic, now iconic, gaze of Derek Bentley, hanged in 1953 at the age of 19 for a murder he didn't commit, stares out of our newspapers and television sets down the years.

On Thursday I watched the news at 9 p.m. with an Australian friend who knew my interest in the case. I had ghosted the memoirs of Derek's sister, Iris Bentley, who had campaigned all her life to clear his name. She died in January.

It had just been announced that "Derek's case", as Iris called it, was finally being referred back to the Court of Appeal, to be heard early next year. But what can it matter now, my friend asked.

It was hard to explain how deeply Derek Bentley's death had etched itself into the British psyche. Hangings were not that unusual then, with executions averaging about eight a year. A few names survive, some because their innocence later emerged - Timothy Evans, James Hanratty. Only Derek Bentley's execution was seen as horrific at the time.

Tens of thousands of ordinary people petitioned the home secretary. The night before the hanging there were riots outside Wandsworth Prison; a mob marched on the Houses of Parliament.

That the trial had been a travesty was not common knowledge (although it is the conduct of the trial that has ensured, 44 years later, that the case will be referred back to the Court of Appeal). What was known was that Bentley hadn't pulled the trigger that killed PC Sydney Miles on the night of a bungled burglary of a confectionery warehouse. He didn't even have a gun and was under arrest at the time the shot was fired, while Christopher Craig, the boy responsible, was 16 and too young to die (he is still alive.)

The truth was even worse. Bentley was an epileptic with a mental age of 11. This was never made known to the jury. The words "Let him have it" that have echoed down through history, words that the police claimed Bentley shouted to goad Craig into firing the gun and thus locked him into the murder, were never said.

They were "borrowed" by the police from an earlier murder case (Rex v Appleby) quoted in Moriarty's Police Law. Yet the myth survives. On Thursday's news, the "Let him have it" chestnut was pulled out of the embers yet again.

There was much controversy at the time, Peter Sissons said, over the ambiguity of the phrase which could have meant "Give him the gun". Not so. This possibility was never discussed in court, because both Craig and Bentley denied the words were ever said.

Forty-four years on and it looks as if Derek Bentley will finally be granted a free pardon. The conduct of the trial alone will ensure the conviction is quashed. However, it was not public opinion or the good will of lawyers that kept the campaign alive but the tenacity of Bentley's family, particularly his sister Iris.

Writing her story was a grim and difficult business. She wanted a Persil-white Derek. I wanted the whole truth. Wishful thinking, I told her, would not do.

Iris was seven when war broke out, and saddled with endless bombs and evacuations, not to mention less-than-reliable parents and a sub-normal brother. Her education was far down the list of family priorities. Yet like everyone else she gathered around her I was won over by her commitment, generosity, humour and vulnerability.

One critic of the celebrity nature of Iris's crusade described her to me as "the regimental goat of the anti-hanging brigade" to be wheeled out at every occasion. There was an element of truth in this jibe. As every TV producer knows, human beings are prurient creatures, and on a scale of prurience the sister of a hanged man rates high.

Her appearance on a panel always gave pro-hangers something tangible to joust against. She was never bitter. And in spite of some appalling provocation she never lost her dignity.

So what has led to the British establishment's change of heart? As recently as 1992 Kenneth Clarke as home secretary concluded: "There are no grounds to recommend that a free pardon should be granted . . . nor to commission a new inquiry into the case."

The problem has always been that initiating inquiries and issuing pardons were the responsibility of the Home Office. But so, too, was the justice system itself, and the police. It was pressure from the Police Federation in 1953 that ensured Derek Bentley hanged. The newly formed Criminal Cases Review Commission, set up this April, is quite independent of the Home Office. It has no considerations other than justice.

Let Him Have Justice by Iris Bentley with Penelope Dening is published by Pan at £5.99 sterling