To passers-by who saw them arguing in the street, it might have seemed like a typical row between a teenager and his mother.

However, this was the moment when Bristol schoolboy Duncan Harrison almost walked out of the television series The Speaker.

Duncan, 15, is now able to laugh about the furious exchange, after he was declared winner of the BBC2 show to find the best young speaker in Britain.

But at the time, he was convinced he was doing so badly that he wanted to give up and go home.

"I'd picked a difficult subject for my speech in the semi-final, and things hadn't been going well. I told mum I wanted to pull out. "Things got a bit heated, although I wouldn't exactly say we had a blazing row."

His mother, Helen, interjects. "I would!" she exclaims. "

"It was a Friday night and we'd gone to Kensington High Street for something to eat. Duncan knew things were going badly for him, and he did the typical teenage thing of saying 'I don't want to do it any more'.

"At that point I was exhausted, because I never sleep properly in hotels. We had a massive row in the street, which is something I probably wouldn't have done if we'd been in the middle of Broadmead, but no-one knew us in London so I didn't care.

"Duncan was saying: 'I want to go home. I want my life back'. He asked me to phone up and say he didn't want to be on the show, but I told him if he wanted to leave he'd have to make that call himself.

"I was nearly in tears. Duncan stormed off. Then a couple of minutes later he came back and said 'If I'd made that call, I'd have regretted it for the rest of my life, wouldn't I?'

"We went to have something to eat and he calmed down, and we began to talk about it. Six months earlier he wouldn't have been able to do that. But he had matured so much from the child who entered the show."

Duncan, a pupil at St Mary Redcliffe and Temple School, was one of 1,500 schoolchildren aged between 14 and 18 from all over the country who entered The Speaker last July.

The show was filmed when Duncan was 14, and he was the youngest contestant to reach the top eight and receive mentoring from public figures including Tony Blair's former director of communications Alastair Campbell, and newsreader Kate Silverton.

"It started off as a bit of fun," recalls Duncan, who appeared in the final – which included a trip to Malawi in Africa – with two other contestants, including fellow St Mary Redcliffe pupil Irene Carter, 17.

"But by the time we got to the semi-final we knew what the final would entail. We all knew what we were playing for. There was a lot at stake, and a lot of pressure."

So how did Duncan manage to win, after being one of the three contestants in danger of being voted off the show during the semi-final?

"I suppose that's where his talent comes in," says Helen, 55, as we sit at the dining table of the family home in Kellaway Avenue, in Horfield.

Until the show, Duncan had never taken part in public speaking events, although he had appeared in a number of stage and television productions, including starring in Oliver! at the Playhouse Theatre in Weston-super-Mare when he was 10.

Duncan started doing classes at Bristol's Helen O'Grady Drama Academy when he was about four, together with his older brother Angus, who is now 17.

Both boys went on to join Bristol School of Performing Arts, and are now members of the Actors' Workshop. Angus is filming a new CBBC comedy drama called My Almost Famous Family at Pinewood Studios.

"Angus is very much the actor, and Duncan is the entertainer," observes Helen. "They've both got fabulous imaginations. When they were little they had a huge dressing up box, and they were always making up their own shows."

What started as a hobby for Angus and Duncan has become an integral part of family life for Helen and her husband Robert, 56, who is manager of Bristol Central Library.

Helen and Robert met when they both worked at the library. However, Helen has had to find other work to fit in with the demands of their sons' acting careers.

In a corner of the sitting room are plastic laundry baskets full of clothes. "I take in ironing to help make ends meet, and I also do work as a film and TV chaperone," she says.

"I couldn't do a nine-to-five job. For the past few years, I've needed to be able to go with the boys to auditions at short notice. It's not so bad now Angus is older, but I still wouldn't let Duncan travel on his own to London."

So what gave Duncan the star quality that set him apart from the other contestants in The Speaker?

Angus, who is studying A-levels at Redland Green School, says one of his brother's strengths is the way he learns from his mistakes.

"Even within an episode you could see Duncan developing," he says. "He might do badly in the beginning, and then take the criticism on board, learn from it, and do really well."

When Duncan replies, I get an insight into the compelling verbal style that won him The Speaker.

"There's no such thing as star quality!" he declares. That's just an old-fashioned Hollywood phrase which is irrelevant nowadays."

I disagree. But Duncan makes his point with such passion that I find that he has persuaded me to contemplate his viewpoint – just like any good speaker should.