Forty-two years have passed since Bruce Balden’s first television appearance, as a fresh-faced seven-year-old whose ambition was to go to Africa and “try to teach people who are not civilised to be, more or less, good”.

Hearing those words today is, he admits, “a bit embarrassing”. But there is something in them that could convince you of the Jesuit maxim “give me the child until he is seven and I will show you the man” - the founding principle of the Seven Up series of documentaries which have followed Bruce’s life and those of 13 others since the early Sixties.

In the third film, as a 21-year-old Oxford undergraduate, Bruce had vague plans to be a mapmaker, but was certain about what he didn’t want to do. “I won’t carry on with maths,” he said firmly. “I don’t think I’ll be a teacher.” By the mid-1980s, he was in a job he didn’t like in the City when he saw an advertisement for a teacher in an east London school.

“In those days there was such a shortage of maths teachers I didn’t make an application,” he says. “I just phoned up and was asked could I go along. I was never really interviewed as such, but the next day the headteacher phoned up and asked if I wanted the job.”

Bruce was untrained and totally unprepared for what lay in store. “It was surreal, really. On the second day a boy asked me what a quadrilateral was and I said ‘that’s easy, just remember your Latin’. He looked at me absolutely blankly. I marched into the staffroom afterwards and said, ‘Can I see the head of classics? This is outrageous.’” He laughs at his naivety.

Gradually, Bruce realised his vocation was indeed “to teach people”. Michael Apted’s absorbing series of documentaries showed him aged 28 starting out as a teacher, living in Bangladesh seven years later, and at 42 back teaching in the East End, newly married to Penny, a history teacher. He came across as patient and committed, at one point saying, “I’m an optimist. I think we can show the way (to) a harmonious multicultural society.”

Over the past seven years, a lot has changed for Bruce. He has become a father of two boys, and has left London for the genteel surroundings of an independent school in Hertfordshire. He says the move was pragmatic rather than ideological, more about seeking a better environment in which to bring up his children than giving up on the state sector.

“What I’m doing now is more of a job than a vocation, but it has its rewards. Expectations are much higher; you have to provide a service to parents who are paying, so there’s a slightly different emphasis to the teaching. It’s still challenging, because I have to try to get As and A*s from kids who aren’t very mathematical but are willing to work hard.”

In an early programme Bruce described private education as “socially divisive”, but now he says: “I think I have probably changed a bit politically. I was in the Labour party, but I didn’t like Blair from the word go so I left in 1995.”

Teaching in Tower Hamlets, he says, “was an education in itself. Going to boarding school, public school and Oxford - my breadth of experience and of meeting people was quite narrow. I have learned a whole different side of life which would have passed me by. I am much more knowledgeable and wiser for having experienced all that.”

But Bruce’s story also epitomises the frustrations felt by many dedicated teachers in inner-city schools. “I was finding it harder and harder to keep going,” he says. “I was getting ground down.”

As he contemplated leaving the East End, he drew up a list of the pros and cons of 18 years spent teaching in one of Britain’s most deprived boroughs.

Highlights included “helping pupils become the first in their family to go to university” and school trips. The list of debits began: “Attacked by pupil; punched in street; being set upon while trying to break up a racist fight; an eight-year abusive telephone campaign; burgled by two of my tutor group; having potatoes hurled at me from tower block; being driven into while cycling; being sworn at repeatedly; having my classroom vandalised...”

One day, while buying a ticket in Old Street underground station, a group of youths began pelting him with gravel. Looking round he recognised three former pupils. He didn’t retaliate but hurried down to the platform.

“A bloke I didn’t know came up to me and said, ‘You’re Mr Balden aren’t you? My son’s just started at university. You taught him in his first year.

He came back really excited after that first week because he had done a piece of work and you had said that it was university material. From that point on he told all his friends he was going to university and did everything he had to do to get there.’

“That’s what it was like; you had to take the rough with the smooth.” In the end, like many in his position, Bruce decided he had taken enough.

Part two of 49 Up is on ITV1 on Thursday September 22 at 9pm