Today’s top public schools are plush country clubs with superb facilities, lovely food, first-class teaching, no fagging, no beating and, one imagines, minimal sexual interference from the staff. Most even have things called girls. While excellent at turning out world-class actors, the public schools these days are far too nice and unbrutal to be of any use as dramatic material for a play.

Julian Mitchell’s play Another Country (1981) belongs to another era. It is a tale of sadistic, crumpet-munching prefects lording it over traumatised fags; homosexuality is rife and there’s brutal jockeying for position among the prefects — all good training for the cabinet jobs these teenagers one day expect to enjoy. It is set in the privileged school system of the 1930s that was briefly a crèche for budding Marxists and traitors. Now the hit play is coming back to London, to the Trafalgar Studios, having been revived to acclaim in Chichester last year.

In the theatre business the play is a legend, having launched the careers of several pimply actors in their very first jobs, including Kenneth Branagh, Rupert Everett,  Daniel Day-Lewis and Colin Firth. Originally it ran for an epic 19 months in the West End (next-door to another tale of public-school japes set in the same period, Daisy Pulls It Off) before a long tour. Funny and sad, the intensely English play hit a chord, evoking public-school life and showing two subversive boys wrecking it.

‘I wrote it,’ says Julian Mitchell, now a spry and entertaining 79-year-old, ‘immediately after Mrs Thatcher’s denunciation of Anthony Blunt, who was in hiding with some friends of mine. Everyone said how it was easy to be a communist in the 1930s — the hunger marches, the unemployment, etc. — but there’s a huge difference between being left-wing and actually wanting to betray your country. Blunt was an absolute lizard, but I thought all the journalists missed something. I thought that the roots of this betrayal might be found in public-school life. They didn’t make the connection between the gayness and the treason.’

The result was a play about school featuring the outrageous, witty Bennett lusting after the pretty boys across the quad through his binoculars. He was loosely based on the Old Etonian spy Guy Burgess. His best friend Tommy Judd reads Das Kapital by torch after lights-out in the dorm and is a stoical, committed young communist. Both are fantastic parts. Burgess, played by Alan Bates, would provide Alan Bennett with the subject of his later film An Englishman Abroad, set in Moscow. Burgess’s crime was memorably summed up by the actress Coral Browne: ‘You pissed in our soup, and we drank it.’

Mitchell is gay. When he went to Winchester in the late Forties he hated it, largely thanks to the ‘fascist’ prefects who ran his house. ‘There was a sadist prefect when I arrived there. The system allowed bullies to get power and they could make people’s lives horrible.’ Young Mitchell was shavered for letting money fall out of his fag master’s trousers when he hung them up; a shavering being a downward cut of the cane used for misdemeanours that didn’t require a full thrashing. It turned him into a writer.

‘I based it on my fury and anger and I wrote it fast and it flowed,’ says its author. ‘It seems to work still. It isn’t set at Winchester; I invented the school. I was trying to write about how a communist and a gay person might have turned against their society to the degree that they utterly hated it. The character of Judd is based on John Cornford, who was from a great Cambridge family of intellectuals and was killed in the Spanish civil war.’

Unlike the public-school Sixties fantasy film If… (in which the staff are machine-gunned by rebel sixth-formers) there are no masters in the play. ‘In my day the boys ran the school, not the masters,’ says Mitchell. The only adult in the cast is an uncle who visits for tea, now played by Julian Wadham, who was a boy in the original cast having been at Ampleforth with Rupert Everett.  The current production stars the up-and-coming actor Rob Callender (a Wykehamist) as Bennett and Will Attenborough (the grandson of Richard Attenborough) as Judd.

Thirty years ago Rupert Everett was replaced by the older, less funny Daniel Day-Lewis as Bennett. Then it was Colin Firth’s turn in the part. When the film was made in 1984, Everett returned to his old part and Colin Firth got to play the cricket-loving communist Judd. Mitchell couldn’t believe his luck that he had turned his unhappy schooldays into a play that gave so much pleasure.

‘When I left school I thought I would grow out of what we all regarded as a “passing phase”. When I discovered I wasn’t going to, I was very upset. I didn’t know how to behave; I didn’t know it was going to be my life. My father hated the fact I was queer. We never discussed it but it was obvious from the things I wrote. He didn’t speak to me for two years after one of my books [his 1968 novel The Undiscovered Country] made things pretty plain. It was very painful for my mother but eventually we made it up.’

The play’s title, incidentally, comes not from Marlowe’s line about fornication (‘but that was in another country/ And besides, the wench is dead’) but from that teary hymn ‘I Vow To Thee, My Country’. It was written by the diplomat Cecil Spring Rice in 1908 as a poem. He later changed it to allude to the terrible British losses in the first world war. ‘It was sung at my prep school at the Remembrance Day service and it was always a very emotional occasion,’ says Mitchell. ‘People never guess the point about it. The line “and there’s another country…” is of course referring to heaven. But to Judd it’s Soviet Russia.’

Mitchell is happy to be a one-hit wonder and delighted to be having his play revived. But who, politically speaking, is the key character? Is it the dangerously disaffected Bennett or his idealist chum? ‘Ah,’ he says, clearly fond of all the boys he created. ‘Watch out for Menzies, he ends up as David Cameron. He’s the one who betrays them all.’