Eighty seems a fashionable age in the theatre this year. Ian McKellen, Alan Ayckbourn and Peter Gill, who has his birthday this weekend, are among the new octogenarians. While the first two are hyperactive and justly lauded, Gill is one of British theatre’s unsung heroes as both a writer and director. Yet his record is remarkable. Among his 18 plays, The York Realist has achieved the status of a modern classic. As a director, he rescued DH Lawrence from theatrical oblivion, established the Riverside Studios as a major venue and turned the National Theatre Studio into the building’s invaluable engine room. His influence on British theatre has been profound.

It would be easy to stick labels on Gill and define him as Welsh, Catholic and gay. But, while those elements are all important, what most strikes me about Gill is his belief in the ultimate seriousness of theatre: something that stems from his early years at the Royal Court and that permeates him like the brand name in a stick of rock. He has picked out the moment that shaped forever his approach to directing. In a fascinating book, Apprenticeship, Gill recalls working as an actor in a 1962 RSC production of Brecht’s A Caucasian Chalk Circle, directed by the Court’s William Gaskill.

Breathtaking … The York Realist at the Donmar Warehouse. Photograph: Johan Persson

It was not a happy experience. But Gill vividly describes how, as part of the preparatory work, Gaskill explored the scene between the two murderers of Clarence in Richard III. Instead of portraying the pair as generalised low-life villains, Gaskill treated them as working professionals for whom killing was a trade. As Gill says, the “empiric elucidation of the scene” and its investigation of its class and political overtones showed him the potential of working in the theatre and influenced his own approach when, in 1965, he turned to directing.

Gill made his debut as a director with a production at the Court of DH Lawrence’s then unknown A Collier’s Friday Night. It was so outstanding that Gill went on in 1968 to revive the complete Lawrence trilogy of mining plays. What I remember is the clarity of the productions and what Gaskill called Gill’s ability “to take a piece of human activity and focus on it with such care that it acquires a luminous life beyond its function”. A key example would be the ritual of washing. In The Daughter-in-Law, the dialogue stopped while a miner back from the pits thoroughly cleansed himself. In A Collier’s Friday Night, an old man took a bath in front of the fire. The action reached its apogee in The Widowing of Mrs Holroyd when a mother and wife washed Holroyd’s dead body with a meticulousness that was both practical and loving.

Today, directors are increasingly judged by their ability to reimagine plays: typically, when the National Theatre revived the Lawrence trilogy they senselessly conflated them into a single work. Gill belongs to an older tradition that not only respects a writer’s text but also seeks to find a style appropriate to the play. A classic example would be his 1984 revival of Otway’s Restoration tragedy, Venice Preserv’d, at the National Theatre. On the one hand, he encouraged his lead actors – McKellen, Michael Pennington and Jane Lapotaire – to perform with the gestural flourish appropriate to a late-17th-century work. At the same time, he brought out the homoeroticism that lay behind the play’s revolutionary politics.

Andrew Scott in Original Sin, directed by Gill, at the Sheffield Crucible in 2002. Photograph: Simon Warner

Gill’s success as a director has tended to overshadow his skill as a dramatist, although in 2002 Michael Grandage staged a season of his work at the Sheffield Crucible that reminded us of his virtues: his excavation of his Cardiff upbringing, his exploration of love and loss, his powerful sense of feeling. You see all that in his first play, The Sleepers Den, which I’ve just reread and which left me overwhelmed. Put very simply, the play is about three generations of Welsh working-class women afflicted by poverty and hardship. It sounds grim but there is a strange beauty about the image of a grandmother, daughter and grandchild nested in bed together. Behind the seemingly naturalistic dialogue lurks a Chekhovian sense of wasted potential. When Mrs Shannon – memorably played by Eileen Atkins in the original production – says of her dying mother, “She’s a woman. She’s been a woman,” you get a sense of a life only partially lived.

Many fine plays – including Small Change, Kick for Touch and Mean Tears – followed, but it was with The York Realist in 2002 that Gill finally achieved the recognition he deserved. It is a breathtaking piece that, dealing with an affair between a farm worker and a metropolitan director during a production of the York Mystery plays, shows the stranglehold on English life of class, culture and roots. It also confirmed Gill’s great strengths as a writer and director: his emotional power, his social awareness and his ability to find rich meanings in the daily business of life.