For some Radio 4 listeners, it was when Sandie Shaw accused fellow 60s singer Jackie Trent of trying to drown her in a swimming pool. For others it was hearing a crew member from HMS Sheffield describe the moment an Exocet missile slammed into the ship during the Falklands War. Or perhaps the shouting-match between former BBC journalist Andrew Gilligan and defence secretary Geoff Hoon as they recalled the Hutton Inquiry into the Government’s “dodgy dossier” on Iraq’s non-existent weapons of mass destruction. Or when former beauty queen Jennifer Hosten met the Women’s Liberation protesters who flour-bombed the Royal Albert Hall in 1970 and brought her crowning as Miss World to a messy halt.

Fans of The Reunion will all have their own favourite moment. The programme started life in 2003, on Sunday mornings when Desert Island Discs was off the air. This Sunday, that prancing little signature tune will announce its 150th edition.

The format, devised by David Prest of Whistledown Productions, is simple. Five people sit around a table and recall an event in which they played key roles. Reunions have covered historical events (the Festival of Britain, the siege of Sarajevo), cultural milestones (the founding of the National Theatre), political activism (Greenham Common), sporting triumphs (the 1966 World Cup) and disasters. The Reunion has been at its most moving when it has covered Hillsborough, Zeebrugge, Dunblane and Omagh.

The guests’ conversations have ranged from heated, angry and argumentative to fond, joshing and sweet, but they’re never boring. Mainly because the presenter holding things together through all 150 programmes has been Sue MacGregor, 76, whose calm and incisive tones steered Woman’s Hour, and later the Today programme, from 1972 to 2002.

Is it a series that thrives on argument? “We’re not a confrontational programme,” MacGregor says when we meet at her north London home. “And I’m not a confrontational chair. But sometimes there are people who have raw edges when they see somebody they haven’t seen for 20 years. You get a lot of tension in the studio.”

Such as? “When we did the fall of Barings Bank, we got both Nick Leeson, the rogue trader chap in Singapore, and his London boss, Peter Norris. Both told the researcher they didn’t think the other would do the show, since they hadn’t spoken since the bank was brought down 16 years earlier. When they met, the atmosphere was tight, but they did address each other.”

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The very first episode, on 27 July 2003, gathered Professor Robert Edwards and four members of the team – two nurses, a gynaecologist and a research student – whose research 25 years before had given the world the very first “test tube baby”. The 150th show, by stark contrast, will feature five survivors of Auschwitz.

Pressed to name her favourite memories, MacGregor recalls some moments of high emotion: “In The Reunion on the anti-hunting Bill, we had a woman whose livelihood was keeping a pack of hounds for hunting. She became so impassioned that tears were pouring down her cheeks.

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“And when the Church of England started admitting women priests, it was difficult to find a communicant who disagreed with their decision. But they found Ann Widdecombe, who was converting to being a Catholic. In the studio she was pretty indignant at being the only person who was against the decision, and very nearly walked out.”

Occasionally they’ve gone abroad – last year to Orlando, Florida, to discuss the 1986 Challenger space shuttle disaster. “What emerged from that was that Nasa had been warned not to take off if the temperature was unusually cold because it would affect the rocket boosters. They took a risk. So the disaster was preventable.”

MacGregor’s periwinkleblue eyes blink with emotion. “It was a heartbreaking programme. I asked June Scobee Rodgers, widow of the shuttle’s commander Dick Scobee, ‘Are you OK to talk about how you watched what was happening, which included the death of your husband?’ and she said, ‘Yes, I’ve talked about it before. It looked at first like a very beautiful thing. But I knew that wasn’t meant to happen…’”

Back in 2009 The Reunion went to South Africa for an edition on the secret talks that led to the release of Nelson Mandela. “We persuaded [then head of intelligence] Niel Barnard, an Afrikaner, to sit down with Archbishop Tutu and the famously liberal editor Allister Sparks. I enjoyed doing it because I knew the territory well.” Between 1962 and 1967, MacGregor had worked at the South African Broadcasting Company in Cape Town.

“Back then there were no people of colour on the staff except the people who swept the floor or worked in the canteen. And when I went back in 2009, everything had changed – everyone was a person of colour, which was great. I suddenly felt, Whew!” And she smiles broadly at this touching reunion – her own, with her younger self, in a different world.

John Walsh

The Reunion airs on Sundays at 11.15am on Radio 4 FM