Alan Bennett has reason to fear birthdays. In Untold Stories – one of his two best-selling volumes of memoirs and diaries – he described spending his 58th in an Italian clinic after a violent mugging while on holiday.
Characteristically of Bennett's wryly autobiographical writing, that memory then triggers reflections on his conception (both he and his brother have 9 May birthdays, possibly because his parents achieved erotic abandon on August bank holiday) and a string of anecdotes about other overshadowed anniversaries.
He celebrated his 40th at the funeral of the father of his friend, the broadcaster Russell Harty; was in hospital for his 46th after suffering an allergic reaction to aspirin; found broken glass in a restaurant salad on his 50th and marked his 54th at the funeral of Miss Shepherd, an eccentric vagrant who took up residence on his north London driveway and was played by Maggie Smith in one of his most successful plays, The Lady in the Van.
And, although his devoted readers and theatre followers will hope he gets through his 80th on Friday without needing to endure either obsequies or a doctor, the event will not be without tremors for Bennett. Because a man who has read Winnie the Pooh on BBC Radio 4 and recently topped a poll to choose Britain's favourite modern play (The History Boys) is at severe risk, with this particular number of candles on the cake, of being elected to membership of the club that he most dreads joining: "national treasures".
In a recent interview, Bennett told me his "eyes narrowed" when he heard the term because it suggests someone soft, innocuous, conventional. And this dislike of being liked too much is the paradox at the core of the writer's work and personality. While his political views have generally been close to those of the Guardian, Bennett has been most enthusiastically lauded by the audience and editorials of the Daily Telegraph.
There are two reasons for this ideological confusion. Because of his prolific presence as a performer – from the Oxbridge revue Beyond the Fringe to roles in his own TV and stage plays – Bennett's face and voice are more familiar to the public than those of any British dramatist since Noel Coward: his plaintive Yorkshire tones and his tweed jacket with sweater and tie are as recognisable an authorial uniform as Coward's drawl and dinner jacket. But, while Coward's persona became associated with confident poshness, Bennett is connected with shy niceness.
This soft manner – and the championing of traditional Anglican churches and service books, the National Gallery and grammar schools – has led to his work being seen as cosier and more conservative than is usually the case. The History Boys contains one of modern literature's few sympathetic pictures of a paedophile – the students are undamaged by being touched up by their teacher Hector – and the playwright has also been unusually tolerant of the traitors Kim Philby (in The Old Country) and Anthony Blunt (in A Question of Attribution).
Indeed, Bennett's later work has often seemed a systematic attempt to unsettle or jettison his core following. His 2012 stage play, People, satirised the National Trust and the heritage industry, while his prose fiction has featured increasingly explicit and kinky sex, with one book bluntly called Smut.
These recent productions and publications, though, are notable for not only content but volume.
Bennett has notably avoided the standard trajectory for dramatists, in which the later decades consist of silence or decline, occasionally broken by revivals of their early successes. On his 80th birthday, Sir Arnold Wesker (author of the Roots trilogy) had endured more than 20 years without a premiere at a major UK theatre, while Sir Peter Shaffer (Amadeus, Equus), celebrating his eight decades, had produced no new work for 14 years.
Bennett differs from Sir Arnold and Sir Peter both nominally – he has bashfully admitted to turning down a knighthood – and productively. In an almost unprecedented creative CV, his best and best-liked plays and books have all appeared since his mid 50s: the magnificent Talking Heads monologues on TV, The Madness of George III, The History Boys and Cocktail Sticks on stage, and the autobiographical collections Writing Home and Untold Stories.
Bennett has acknowledged being a late starter artistically and sexually; he now lives with his civil partner, Rupert Thomas. But, at 80, he is nationally visible – with birthday seasons of his work on the BBC and at the West Yorkshire Playhouse – and personally and professionally treasured. Just don't call him a national treasure.
• B is for Betty's, S is for splother ... An A-Z of Alan Bennett
• 80 years on: Alan Bennett's career in pictures
• This article was amended on 9 May 2014. An earlier version referred to Beyond the Fringe as an Oxford University revue. Two members of the four-strong cast were from Cambridge.