I begin with a chuckle that turns into a lament. The Pickerskill Reports, the series in which a former public school master reminisces in a tone as lacerating as it is fond, is no more. Author and director Andrew McGibbon has declared it's a wrap with The Last Report, bringing bank-holiday merriment with its mix of the scholarly and the fantastical in a one-off special.

The series originally starred Ian Richardson, whose recruitment was a coup in itself and whose performance so matched the withering put-downs and acute insights of the script that all seemed lost for the show after his death in 2007.

Yet producer Curtains For Radio brought the series back in 2009 with another casting triumph - with his reedy-voiced enunciation and donnish demeanour, Ian McDiarmid stepped effortlessly into the other Ian's footsteps. This new play finds McDiarmid in full flight, relishing the effortless dialogue ("brandishing your braggadocio"), high-table erudition and gentle, literate comedy that can turn savage in a second.

As the 'progressive' head, Michael Feast fuses the sinister and the cynical, while Tony Gardner is the spluttering maths master Lefty, around whom Thomas Brodie-Sangster's precocious pupil, Porter, runs rings. With her girlish politeness, Elaine Cassidy is the antithesis of a villainess - until she is revealed to be working for the kind of suspect organisation beloved of 1960's TV series such as The Avengers.

Before Richardson's death, there was dangerous talk of a transfer to television, but perhaps The Pickerskill Reports is best remembered as one of radio's timeless jewels.

The comedy drama The Pickerskill Reports, in which the retired English master of a boys' school looks back on some of his favourite pupils, files one final report. In this one-off, we hear how orphaned Oliver, who was much bullied by the other boys for being a bit of a know-all, returns to his old school. Oliver's now in the clutches of a cult called The Constancy and has plans for his alma mater. Ian McDiarmid stars in this very odd, but curiously entertaining finale.

Very last episode of the witty social satire, blessed by a superb cast (Ian McDiarmid, Mark Heap and Michael Feast), written and directed by Andrew McGibbon. McDiarmid plays the wily head of a school which has gone through many transformations and whose past pupils duly represent the fact, whether pillars of the establishment, captains of industry or various other grades of dodgy geezer. Now meet Faye (Elaine Cassidy) who will save its site from mercantile exploitation to transform it into a beacon of the new educational ethos.

This long-running black comedy starring Ian McDiarmid as Dr Pickerskill, a retired English master looking back at the lives of his most entertaining pupils at Haunchurst School, draws to a close with a special send-off. In The Final Report, written by Andrew McGibbon, Pickerskill has to contend with a charismatic cult leader called Faye, whose shady sect, The Constancy, is planning to take over the college. Elaine Cassidy and Mark Heap also star.

Since 2005, darkly comic series The Pickerskill Reports has been an important part of Radio 4's schedule. Set just after the Second World War at Haunchurst College, an English public school, it follows the reminiscences of retired English teacher Dr Pickerskill (played by Ian McDiarmid). On Monday 27 May, the series comes to an end with Haunchurst facing the threat of a takeover by cult leader and former pupil Faye Hornette (played by Elaine Cassidy). Daily Telegraph radio critic Gillian Reynolds has praised the series, saying: "The Pickerskill Reports invites you into a very interesting, tightly controlled world. It's very subversive and anarchic but you never feel uncomfortable."

Ian McDiarmid plays Dr Henry Pickerskill, a teacher at Haunchurst College for Boys in the days when it had boarders, icily remembering former pupils. Today, Patrick Trumball (Toby Longworth) who was fascinated by thunderstorms and lightning. What Dr Pickerskill spotted was that Trumball, an unusual child, also possessed a photographic memory which came in handy when the school's dodgy accountant blamed Pickerskill for a fraudulent tax return. Written and produced by Andrew McGibbon, this (all too short) series restores cynicism's good name.

He's part Betjeman, part Meldrew and part stand-up comic gunning for his audience. There can't be many Dr Henry Pickerskills alive and working now in the education system because his brand of tough love, rough justice and abrasive shafts of wit directed at his pupils would probably breach the Human Rights Act.

The Pickerskill Reports is back with more of Dr P's ironic stories of life long ago at a boys' boarding school whose ethos is one in which normal laws don't apply, let alone what would then have been fanciful libertarianism. Dr P arrives in his classroom like Dirty Harry. Nemesis of bullies, alleviator of the torment of those he calls his "spirited iconoclasts" and a complete enigma to his headmaster who, somewhere in the recesses of his brain, knows that his English teacher is running rings around him.

The head is portrayed as militaristic but feeble-minded by Mike Sarne, the actor and director who has covered much ground since he tried to persuade Wendy Richard to Come Outside in the 1962 pop song. Making a guest appearance in the first episode was Sheridan Smith, also showing her versatility as an aristocrat playing at gender politics. Ian McDiarmid, as Dr P, brings a dryness of delivery and impeccable timing to the role while clearly relishing Andrew McGibbon's sardonic and entertaining script.

If a TV soap actor vacates a role, the character is wont to return with a new head on and no questions asked. When Heath Ledger died in the saddle, Hollywood drafted in a triptych of actors to complete the movie. But the sudden demise of Ian Richardson, 18 months after his first radio performance for 20 years, seemed to have spelled an end to the waspish comedy series, The Pickerskill Detentions, which I hailed in 2005 as "glorious".

Certainly, plans by independent producer Curtains for Radio to take it to television were ditched, presumably because only Richardson offered that elusive combination of bankability and credibility. Andrew McGibbon's creation, dark, satiric and with a hint of classical nemesis in every sting in the tail story, was too good to be lost to radio and it has returned in a new series as The Pickerskill Reports, undiminished by time and tragedy.

Ian McDiarmid has stepped brilliantly into the central role of the former schoolmaster, commenting from near and far on the malign activities of his former students, witticisms dispensed with vinegary glee, his plummy satisfaction as he outwits them conveyed with a baleful undertone. Each episode is an account of a different pupil who is outstanding for all the wrong reasons. However slippery the little devils are, they cannot outsmart Pickerskill, who has the forensic abilities of a murder squad veteran and the psychometric testing skills of an occupational psychiatrist.

His first subject is, in Pickerskill's estimation, a sociopath. He soon stumbles on Walter Hindle-Rand's plot to bribe the alcoholic maths master, dubbed the "meths master", with altar wine, in exchange for academic favours. Thomas Sangster played the boy in a performance in which the angelic facade of the choir boy was peeled away to reveal youthful cruelty in action. McGibbon's evocation of a boys' school in the sixties was like The History Boys stripped bare of idealism, a jungly training ground for the world. This was laugh out loud comedy, stabs of truth doled out with the humour.

Don't miss The Pickerskill Reports (Radio 4, Friday mornings). This is comedy done without a studio audience, nothing between listener and performance but the ringing of the doorbell, the noises in the street. It is good enough to blot out both. Ian McDiarmid translates Andrew McGibbons's script into a world of its own, faintly sinister, oddly true and far too funny ever to be on TV.

Ian McDiarmid plays Dr Henry Pickerskill, retired teacher of English at a now defunct boys' public school. What he's always looked out for are oddities, the boys who didn't conform to the solid stereotype, the spirited iconoclasts, prizing them for their originality. That means, of course, the ones with a bent for junior criminality. This is his series of reports on such chaps, what happened at the school and later. It's very funny, in a sinister, sarcastic sort of way, beautifully written (by Andrew McGibbon), and brilliantly played.