The train slides through greasy rain into the industrial entrails of Wolverhampton: the rusting cars, abandoned sidings, huge weeds, terraced houses, graffiti – and past the filthy canal where Sathnam Sanghera threw the plait of black hair that once hung down to his knees. In a cloistered Sikh-immigrant community, chopping off hair that had never seen scissors was no mere gesture. It was the beginning of a journey during which Sanghera pulled up his roots, examined them and delved deep into his family’s secret history.

Sanghera was 14 when he divested himself of his topknot and a future turban. It has taken him until today, aged 31, to understand his troubled past, uncover the schizophrenia that ran through his family and evade plans for an arranged marriage. He is now an urbane, trendy business journalist, with a column in The Times, and his memoir, If You Don’t Know Me By Now (and I speak without corporate bias), is funny, agonizing and moving – a relief in these days of ghost-written, crapulous “misery memoirs”.

“By the time I was eight,” he writes, “I had never been to a cinema, used a telephone, been inside a church, used a shower, sat in a bath – we still used a bucket and jug – seen the countryside or the sea, read a newspaper, had a white friend, owned a book, met a Muslim or a Tory or a Jew.”

The book tells us a great deal about Sanghera – a boy who worked part-time for 50p an hour in a sewing factory from the age of ten and then went on to Wolverhampton Grammar and Cambridge. But it also tells us how much – and how little – Britain has changed in the 40 years since one-time Wolverhampton MP Enoch Powell made his infamous “Rivers of Blood” speech.

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Sanghera picks me up at Wolverhampton rail/bus station in a black Kia estate car. “It isn’t mine,” he defends himself, boyishly. “I’m reviewing it for a magazine.” He shoots round a tight corner. “My mother usually prays when I drive.”

His mother, Surjit, whom we shall meet later, speaks almost no English, a reflection of the closeness of the Punjabi community, “isolated by language and illiteracy and focused on survival”, who arrived from India in the late Sixties. When Sanghera takes me to where he grew up in Park Village – neither a park nor a village; more Coronation Street gone slummy – you can see how claustrophobic his world once was: the poky terraced houses with doors directly on to the street, the Sikh-run pub at one end, the corner shop at the other, the temple and Woden Primary School nearby. “Your parents knew within minutes if you’d done something – 24-hour TV news has nothing on the Indian gossip network.”

Yet while everyone knew everyone else’s business, there were matters within the Sanghera family that were never discussed. Not until he was 24 did Sanghera discover that his father, Jagjit, and his sister, Narinder, had been diagnosed as schizophrenics. It explained a lot: why his father sat on the sofa all day watching BBC Parliament and his mother sewed full time and raised a family of four by herself. It explained the day that his sister heard voices, locked herself in a bedroom, and was eventually taken off to mental hospital. (Narinder still lives in Wolverhampton and is now married with two children.)

When Sanghera went back to write the book, exorcise his demons and build up to telling his family he wasn’t going to marry a Sikh girl of their choosing, he started asking his mother what it was like when she came from India and was set up in an arranged marriage with a stranger. What did his father look like when she first met him, he asked. “To be honest, I don’t remember what he looked like,” his mother said eventually. “All I remember is he came up to me and slapped me on the face.”

Sanghera’s mother, deeply religious and bound by the concept of izzat or honour, put up with years of unexplained anger and incredible violence from her husband until his mental illness was finally diagnosed and he was medicated. “It’s hard to tally that history. Now he is the gentlest man you could ever meet. Our family have completely forgiven him. He was ill then,” Sanghera says.

We walk through a passageway strewn with rotting cushions, lost clothes and dead laminate flooring. It leads to the narrow gardens and railway lines behind the two-up two-down houses. “When I was a kid women would take turns at sweeping this passage. And these were vegetable gardens,” he says, pointing at a wasteland of old sofas.

The area still has some Indian families, but many have moved out to the suburbs, leaving Park Village to the latest immigrants, renters from Eastern Europe. On one wall there are a number of crazy signs written by Stan, a local, including: “Pay Attention – Avoid Deth.” Sanghera says: “That was once the polling station. The men would stand outside and say, ‘Vote Labour, mark number two box’, because no one could read the names.”

Other childhood memories are the compulsory four hours of sitting crosslegged at the temple every Sunday. We go there, into a plain brick building draped with strings of flags, cover our heads and pad across the thick carpet to bow before the altar and make our offerings in a golden trough. “There would be 200 people in here. It would be boiling, and there were no fans. I couldn’t understand what they were saying. I asked my uncle and he said he didn’t understand some of it either. There were some hymns in Sanskrit,” says Sanghera, who has fashioned himself a splendid makeshift turban out of sparkly purple cloth. His mother would bring him here at dawn on his birthday to clean the temple for three hours “as a special treat”.

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