All Ludovic Kennedy wanted his obituaries to say was: “He tried to do good, but may have failed.” Yet if the carriage of justice in Britain is now safer than it once was, it is thanks in significant part to his vigour and persistence.

Best known by the public as a television performer, for 50 years he also investigated wrongful convictions and campaigned in print and on screen on behalf of the victims. He was the scourge of lying police officers and complacent judges. His revelations opened eyes to the failings and corruptions in the legal system. He liked to quote Goethe: “Distrust all those in whom the urge to punish is strong.”

This pursuit of the powerful might seem surprising from a convivial clubman with an impeccable establishment background – Eton and Oxford – but there was always an innocence and a touch of unworldliness about Kennedy.

On holiday as a boy with his grandfather, a law professor, he immersed himself in volumes of Notable British Trials. He was impressed by “the majesty, mystique and above all the accuracy of the law”, but great was his disillusion when as a journalist he discovered British justice was not as he had believed.

His passion for righting wrongs may have started with his family. Born in Edinburgh in 1919, he came to hate his domineering mother but was devoted to his naval officer father, who was reprimanded after an unjust court martial. Years later in 1939 Captain Edward Kennedy, aged 60, was recalled to command the armed merchantman HMS Rawalpindi, later sunk in a courageous engagement with the German ship, the Scharnhorst. Ludo, himself in the navy by then, served on a destroyer – he described the four days chasing and watching the sinking of the German battleship, the Bismarck, as the most memorable of his life.

After the war he started writing, and married the beautiful ballerina Moira Shearer. They had three daughters and a son. Now well known as the dashing husband of the famous dancer, he grew more so when he began his TV career in the mid-1950s. After failing to win a seat as an MP for the Liberal party, he moved to the BBC’s flagship current affairs programme, Panorama. He went on to present 24 Hours, a current affairs show .

Ludo – as he was always called – brought intelligence, eloquence and an urbane charm to all his screen work. He appeared so relaxed, some claimed to have seen him napping, though he insisted: “I only ever fell asleep in the studio once.” He took television seriously but with a raised eyebrow. Of a controversial Panorama he wrote: “In a few days, like everything else on television, it was forgotten.”

A critic commented that whenever he saw Kennedy on TV, he gave the impression that he had been “good enough to drop by to see if he can lend a hand while on his way to the club”. Kennedy was sufficiently tickled to call his highly readable 1989 autobiography On My Way To the Club.

Despite his apparent ease, however, it was only in the 1970s that he threw off the depression he had suffered for 25 years despite endless visits to psychiatrists and a course of electric shock therapy.

The case that first shook Kennedy’s faith in the legal system was the conviction in 1952 of Christopher Craig and Derek Bentley for the murder of a policeman in London. Craig, 16, had fired the shot, but was too young to hang; 19-year-old Bentley, who had an IQ of 66, was hanged. Kennedy wrote a play based on the case, Murder Story (1954), which ran in London’s West End.

Then came the hugely influential 10 Rillington Place (1961). Kennedy showed that the illiterate Timothy Evans had been wrongly hanged for a murder almost certainly committed by his fellow lodger, the necrophiliac multi-murderer John Christie. After two inquiries, Evans was granted a posthumous free pardon.

There followed The Trial of Stephen Ward (1964) on the osteopath who committed suicide while on trial for living off immoral earnings following the UK’s Profumo scandal; and Presumption of Innocence (1975) on Patrick Meehan, a Scottish burglar eventually freed and compensated after being wrongfully sentenced to life imprisonment for murder. The Scottish legal establishment resented having its errors revealed and when Kennedy was put up for membership of Muirfield golf club, lawyers blackballed him.

Undeterred, Kennedy wrote The Airman and the Carpenter (1985), arguing that Bruno Richard Hauptmann had been framed for the kidnapping and murder of aviator Charles Lindbergh’s baby son in 1932. His evidence was powerful but the case has never been reopened.

Kennedy always argued for an inquisitorial justice system to replace the adversarial one, which he believed to be open to corruption by the investigating police.

A BBC documentary, Who Killed the Lindbergh Baby?, accompanied the book, one of many documentaries Kennedy made over the years, among them Scapa Flow, Battleship Bismarck, Life and Death of the Scharnhorst and Great Railway Journeys.

In his eighties, the handsome, debonair figure became stooped over a silver topped cane but the writing never ceased. Neither did the charm nor the wicked chuckle.

In All In the Mind: A Farewell To God (1999), a blast at all religious belief, he wrote: “For me the end of life is the end of everything. Everybody has to find meaning in their own life, and a lot of people can’t because they are not reflective. I think that’s why a lot of lives are rather unhappy. Through the things I have done, I have found a great deal of meaning in my life. I don’t say that they have been terrifically successful, but they have satisfied me and given my life a purpose.”

Kennedy was knighted in 1994.