The morning after the first episode of Who Wants To Be A Millionaire? transmitted on 4th September 1998, a lorry-driver spotted Chris Tarrant walking through Wembley and cheerfully hollered, “Oi, Chris, phone a friend?”

Twenty-five years later – and ten years after he stopped hosting the all-conquering ITV quiz – Tarrant still gets it.

“Even yesterday,” he laughs. “It’s never a problem, it’s just people’s way of saying they like the show.

“Sometimes they get the catchphrases wrong and say ‘Hello Chris, are you going to phone the audience?’ Eh? How would that work?”

Once, after he ordered a pint in a London pub, the landlord pulled it and poured it straight into the slop bucket saying, “But we don’t want to give you that…”

Being heckled in the streets by smiling strangers was nothing new. “People had been shouting things at me since Tiswas, usually about custard or Sally James,” says Chris, 76, recalling his gloriously anarchic Saturday morning show.

Ostensibly for kids, but loved by adults too, Tiswas with its caged spectators, pie-slinging Phantom Flan Flinger, and hit single (The Bucket Of Water Song) made stars of Tarrant, James and Lenny Henry.

It was an unusual career change for the teacher-turned-TV-newsman.

English graduate Tarrant started teaching at Samuel Pepys County Secondary School in New Cross, southeast London, in 1971. “It was bloody tough,” he tells me. “I taught the Easter leavers – all boys; they’d had enough at 14. Some of them were bigger than me. There were 42 in the class but half of them wouldn’t turn up, they’d be off working with their dads.

“One got arrested for driving a stolen car when he should have been in maths.”

Chris, busy promoting the paperback edition of his laughter-packed memoir, It’s Not A Proper Job, escaped by writing “a dreadful brash letter” to every ITV company, including “the awful phrase, ‘I am the face of the 70s, this is your last chance to snap me up.”

Incredibly three replied, and in 1972 he started work on the Midlands-based news programme ATV Today. Tarrant did serious news initially but inevitably graduated to lighter items – “lightbulb eaters, hedgehog jugglers, men who ate live frogs…”.

His father, Basil – an infantry major veteran of Dunkirk and D-Day – and mother Joan lived near Chris’s home-town of Reading and couldn’t see the show.

“So unknown to me, they booked into a hotel in Oxford to watch their son do fearless hard news reporting, only to see me covering a bloke on a charity walk with four ferrets down his trousers…”

Perfect training for Tiswas.

When ATV wanted someone slightly potty to host the hare-brained new show, his news producer instantly suggested Tarrant. It ran for eight laughter-packed years; guests included Paul McCartney, Queen and Annie Lennox – all fans.

“Frank Carson was on it relentlessly, whether we booked him for it or not. He’d turn up and say, ‘The show was dying, I thought I’d save it.’

“One day he brought a funny little bloke with him who turned out to be Bob Carolgees with Spit The Dog.”

They covered caged adults in gunge years before Noel Edmonds. “People queued up for it,” chuckles Chris. “One woman spent the morning in The Cage but when she watched the video she couldn’t even recognise herself, she was just smothered…”

When The Bucket Of Water Song was a hit, the cast became targets. “People came up to us in pubs and poured beer over our heads, and we always took it, but once in a restaurant in Brighton a terribly posh lady poured the ice bucket over Sally’s head, which was too much.”

Tarrant is still as hearty, congenial and unaffected by fame as he was when I first interviewed him on the set of Tiswas’s doomed 1982 adult spin-off O.T.T.

“We had huge ratings at midnight and Lenny Henry was brilliant on it, he just grew and grew. But it was very experimental and some of it was crap; maybe I didn’t have enough control over it as a producer, we were just trying out ideas live.

“One week we’d have Alexei Sayle, the next Bernard Manning. On a Monday morning the ITV controller used to kick me around the office but we took chances and had great bands on – Deep Purple, Status Quo, The Human League.”

Radio gave him more freedom. “I had the best time of my life at Capital Radio. It was an exhausting 17 years, but we beat off all the upstarts, travelled the world and I had a brilliant time, getting paid to play records and talk nonsense.

“I like being able to think of an idea on the drive to work and just do it. Nowadays everyone has a webcam. The first thing I did at Capital was lob a coat over the camera. I’m only doing radio! Watching people on radio is hugely boring.”

The germ of Who Wants To Be A Millionaire? came from his Capital quiz, Double Or Quits. Tarrant’s producer asked him to front the pilot. By the time they filmed the second, with added dimmed lights and dramatic music, everyone was hooked. ITV boss David Liddiment decided it should go out live every night for ten days.

“There was so much pressure the first night, we had a therapist on stand-by. Someone won £64,000 – nobody had won that kind of money before.”

In its second year, Millionaire filled 19 places in the TV Top 20 – “we smashed the BBC to pieces” – and in 2000 Judith Keppel made world news by becoming the first-ever quizzer to win £1million.

“I thought we might get four years out of the show, I did thirty series! By the end of my time, I’d had enough. ITV wanted more and more celebrity editions and there are only so many famous people. We had Paul McCartney, George Michael, Frederick Forsyth…people who never go on game shows.

“I knew sooner or later they’d use reality TV people. I didn’t want to go down that route.”

Why was it so successful?

“It’s a simple format, it grabbed people, the answer was right in front of you so it had shout-ability. Judy Finnigan said she heard Richard Madeley use language shouting at Millionaire that she’d never heard in all their years of marriage.”

The show’s only scandal was James Imgram – aka the Coughing Major – who was found guilty of cheating his way to the £1million prize.

The saga was re-told as drama on ITV’s Quiz. “Michael Sheen did me, he’s an actor not an impressionist, but some of his mannerisms as me were wonderful. Mike Osman mimics me best, he’s so close, and his Trump is brilliant too. Rory Bremner is going around the country being me. I’ve never gone ‘tee-hee’ in my life.”

He hasn’t seen Jeremy Clarkson’s version. “I get on with him socially, I just don’t watch game shows. I watch Netflix, the news, Sky Sport and Sky Arts.”

Father-of-four Chris lives happily with his third wife Jane in an idyllic Cambridge Village. They’ve been together seventeen years. “She’s my mate,” Tarrant beams.

He had a serious stroke in 2014 flying home from filming Extreme Railway Journeys in Bangkok but says, “I’m all good now. I used to work like a dog but I take it easier. I did a nice thing with John Cleese for his chat show the other day because I’d never met him.

“I’m supposed to be doing a film about bears and trying to do TV with my son, he talks more than I do.

“I don’t drink whisky every day but I still like a beer. I travel a lot. I took my eldest grandchildren on safari in South Africa, their eyes were out on stalks. It was beautiful to watch their reactions.

“I’m trying to get to the Caribbean to see some cricket.

“I’ve been to too many friends’ funerals. I don’t know how long I’ve got but I know I’m going to enjoy every minute.”

• It’s Not A Proper Job by Chris Tarrant (Great Northern Books) is out now in paperback. More details are at gnbooks.co.uk