This is the story of an unlikely friendship. It was a little out of the ordinary when it began, but as the years went by and the friendship deepened, it bumped up against the outer edges of plausibility.

In 1966, a young, badly dressed, gap-toothed man arrived in London for the first time. In a number of respects, he was not unlike thousands of other tourists: he was innocent, curious and had an inexpert – very inexpert – knowledge of English. But in other respects he wasn’t like them at all. For a start, he was a different shape.

As he says himself, ‘I was like this little puppy, you know, venturing out into the world. When I first came to London, I’d never even been on a plane before.’

Aged 19, just out of the Austrian army after doing his national service, Arnold Schwarzenegger had come to London to compete in the Mr Universe competition. His expectations were not high. ‘All I was hoping for was to get into the top six. I’d had to train in this military gym in Austria, which just had a lot of crude dumbbells and barbells. I knew that if I was ever going to get anywhere I would have to have access to much better equipment.’

But when he stepped out onto the stage to show off his voluminous deltoids, his tumescent biceps and his hogshead-like chest, there was a stir of excitement – of disbelief even – among the judges. Two in particular craned forward for a closer look. One of them was Wag Bennett. The owner of a gym in London’s Forest Gate and the first man in Britain to bench-press 500lbs, Bennett knew immediately that this wasn’t just any old beefcake. It was prime cut.

Also in the audience that day was Wag’s wife, Dianne. The daughter of a bodybuilder, Dianne lifted weights herself and had a troupe of muscle-women called Dianne Bennett’s Glamour Girls. ‘I’ve lived my whole life among big men, even when I was a little kid,’ she says. ‘I’ve never been out with a scrawny man. Anyone thin or with a pot belly is just… ugh to me. But I could see straight away that Arnold was something unusual.’

Schwarzenegger ended up coming second, beaten by the American, Chester Yorton. After the competition was over, the Bennetts went up and introduced themselves. ‘Arnold really couldn’t speak English and he just stood there – this big, gauche, Austrian hick who was literally bursting out of his clothes.’

Partly because they felt sorry for him, and partly because they could see his potential, the Bennetts invited him to come and stay in their terraced house in Forest Gate. ‘I remember we explained that we had kids, so he knew that we were… you know, genuine.’

Schwarzenegger eagerly accepted their offer. But with six young children, the Bennetts had no spare bedroom so he dossed on a sofa in the living room. Fortunately, it was a big sofa.

‘I think he liked the informality of our home,’ Dianne recalls. ‘He loved the fact that we all mucked in together. Because Wag and I worked during the evening, we would read the children bedtime stories in the morning. They would all come in and get into bed with us. Arnold would be listening on the other side of the wall and if Wag stopped for any reason, he’d stick his head through the hatch and say, "Vot happened next?" ‘ Wag Bennett died last year, but almost half a century on, Dianne, now 73, is as devoted to bodybuilding – and to Arnold Schwarzenegger – as ever. The devotion is clearly reciprocated. As well as talking regularly to him on the phone, Dianne tries to go over to California once a year.

These days, she lives and works in Portsmouth, running the same gym that her father once owned. All over the walls are photos of the fresh-faced Schwarzenegger, often with his arm around Dianne and Wag and smiling his wide-open smile. In several shots he’s wearing the lime green Crimplene shirts that Dianne used to make specially for him. Too big to fit into normal shop shirts, he liked Crimplene because he didn’t have to iron it.

Dianne Bennett describes the gym as a ‘no bull——’ sort of place, and much the same thing could be said of her. With her swept back blonde hair, her dark glasses and her fiercely uncompromising manner, she looks like she could reduce the most uppity muscleman to nervous whinnies.

‘How many instructors do you have working here?’ I ask as she shows me around.

‘Instructors?’ she says frowning in surprise. ‘Just me.’

A year after he first arrived in London, Schwarzenegger tried to become Mr Universe once again. This time he succeeded. He had now made it onto the first rung of his career ladder – a ladder he would spend hours poring over in the Bennett household.

‘At first, when Arnold would talk about what he intended to do with his life, I tended to take it with a pinch of salt. He’d sit there and explain – albeit in rudimentary English – how he was going to be the greatest bodybuilder of all time, and that he wanted to own real estate and be a movie star. We’d all be going, "Yeah, yeah, yeah… He’s all mouth and no trousers".

‘To be honest, I never thought Arnold was that good looking, although he’s got more handsome as he’s got older – and let’s face it, he’s never been a great actor. But what I came to see was that he had this dynamic, burning desire to succeed. I’ve never come across anyone with that kind of determination before, or since.’

Over the next two years, Schwarzenegger stayed regularly with the Bennetts, often for several weeks at a time. He’d take the children to swim at the local baths and pick them up from school. At mealtimes, Wag Bennett would put two large planks on top of the dining room table so that it would accommodate everyone – he sat at one end while Arnold sat at the other. They grew so close that Schwarzenegger began referring to the Bennetts as his ‘British parents’.

Soon Wag and Dianne were organising shows around the country at which Arnold would strip off, go through his moves, and be paid £10 a time. He also regularly visited the house that Dianne now lives and works in – it belonged to her father before her. Arnold would sleep under the eaves in a bedroom that’s still piled high with bodybuilding magazines.

She taught Schwarzenegger to speak English and encouraged him to read books. ‘He’d never actually read a book before he came to us. Quite often I would send him books that I thought he might find interesting.’ In what now seems a remarkably prescient move, one of the books she sent him was a biography of Joseph Kennedy – father of John F Kennedy and the man who would one day become Schwarzenegger’s (late) grandfather-in-law when he married Maria Shriver.

The Bennetts’ influence was felt in other ways, too. ‘We taught him posing skills and obviously helped his language. We also helped him to articulate the greatness that was inside him. And maybe we encouraged him to develop more mundane characteristics such as liking family life,’ says Dianne.