John Mortimer never tired of describing himself as the best playwright who ever defended a murderer at the Old Bailey. And it was true that the legal world, which he had entered reluctantly, served him well as a writer.

Being a lawyer gave him a cachet, as the only playwright-QC, a subject — many plays, and his most famous character, the barrister Horace Rumpole, had courtroom backgrounds — and a refuge: if a play did not prosper, he could always retreat to chambers and take another brief.

Apart from inventing Rumpole, a deliberate move to create an immortal character like Sherlock Holmes or Maigret “to keep me in my old age”, he won acclaim for radio plays (The Dock Brief took the Prix Italia in 1957); for his autobiographical and nostalgic play A Voyage Round My Father; and for the first of three volumes of memoirs, Clinging to the Wreckage — a true masterpiece of the genre.

His large and sometimes rumpled figure, like that of his Rumpole character, was instantly recognisable; he exuded a universal bonhomie and revelled in a party-going social life even when conducted from the confines of a wheelchair. He liked nothing better, as life went on, than addressing audiences, performing his cabaret act, Mortimer’s Miscellany, so widely that there could hardly be anyone in the land unfamiliar with his oft-told anecdotes, eg, about the hopeless Irish drunk who was let off by a judge if he promised to renounce alcohol: “And I really do mean that: not even the teeniest weeniest little dry sherry before dinner.”

Many of his most amusing stories came from his sometimes irascible father, whose terrifying interrogatives (“Is execution done on Cawdor?”) and aperçus (“All schoolmasters have second-rate minds”) became so familiar from Mortimer’s work that not even Mortimer himself could recall whether his father had actually said them.

John Clifford Mortimer was born in 1923 in Hampstead, the only child of Herbert Clifford Mortimer, a barrister specialising in probate and divorce, and his wife Kathleen. Both had lived in South Africa. The boy was sent to Mr Gibbs’s prep school in Sloane Street (where Peter Ustinov was a fellow pupil) and then to board at the Dragon School, Oxford, whose eccentricities — in particular those of the headmaster, “Hum” Lynam — were guyed in his later plays.

Mortimer always claimed that the opportunity to play Richard II in 1937 in the Dragon’s annual Shakespeare play was the high point in his life, and reviews in The Draconian testify to his outstanding performance. It is odd, therefore, that Mortimer — who spent his solitary childhood constructing sets for his toy theatre, and performing solo versions of Hamlet or Macbeth for his parents—– never again took to the boards. At Harrow he spoke in debates, formed a one-man communist cell, wrote and painted, and by dint of long hours in the Vaughan Library, became extremely well read.

He went up to Brasenose College, Oxford, at 17, to read law at his father’s insistence. With his deteriorating eyesight, Mortimer senior wanted his son in chambers as soon as possible. Mortimer would have been better suited (having, in his housemaster’s words, “bohemian tendencies” and “antinomian views”) to reading English, but his father was adamant.

Billeted in wartime Christ Church, skinny, bespectacled and effete-voiced, he wrote poetry and began to affect outfits in flamboyant purple velvet and to embark on intense friendships, particularly among the aesthetes. His Oxford years ended abruptly in July 1942 when he wrote florid letters to a handsome Bradfield sixth-former visiting Oxford, and the letters were found by the boy’s housemaster. The young man, who later became a distinguished judge himself, was expelled from his school and Mortimer was told by the Dean of Christ Church not to return. This “small scandal” was soon expunged from his memory, but resurfaced in the themes of several plays he later wrote, including one about his schoolday hero, Lord Byron.

Pronounced medically unfit for the Armed Forces, back in the family home, a modest house built by his father in 1932 in the deeply rural Thames Valley, he embarked on his first (unpublished) novel. But he had to fulfil the approved war work requirement to get his degree. Jack Beddington of the Crown Film Unit lived near by: he arranged for 19-year-old Mortimer to join the unit at Pinewood, making propaganda films. This supplied him with a cast of characters and plenty of dialogue and plot to inspire his first novel, Charade, published to high critical praise in 1947.

His father, now blind and dependent on his wife to read his briefs aloud and lead him into court, welcomed his son into No 1 Dr Johnson’s Buildings in the Temple immediately after he was called to the Bar in January 1948. The following year Mortimer married the bewitching Penelope Dimont, née Fletcher, a fellow novelist five years his senior, who was living near by and, when they fell in love, was already pregnant with her fourth daughter. Both Mortimers had novels published that year. So the couple set up home in a Temple flat with a ready-madefamily; their own daughter Sally arrived in 1950.

This teeming, often chaotic household — which moved in 1952 to a more spacious Victorian house at Swiss Cottage — became the setting for a dramatic, volatile, competitive marriage, which two novelists living in 1950s Hampstead were almost bound, by trendy convention, to pursue.