Lin Sampson talks to Katie Nicholl, Royal Correspondent for the Mail on Sunday, about the extraordinary story of two ordinary boys

Katie Nicholl exemplifies the modern European woman, well groomed, wearing monochromatic cashmere and expensive boots and is so gracious she could slip into the Royal family she writes about without a wrinkle, which is exactly what she has done with her book William and Harry.

Nicholl accompanied the princes on their recent trip to South Africa but says she really got to know them in London, where she appears to have spent a large amount of time in high-tone clubs, where the cocktails are called snakebites and the girls have names like Isabella Anstruther-Gough-Calthorpe, one of the first girls to knock back Prince William.

"Honestly, the most insightful times I have had with them is seeing them at clubs or polo matches when I am off the record," she says.

Her route to fame started when she witnessed Harry's partying first hand as a young show business reporter covering an event at the Kensington Roof Gardens. "I had gone outside onto the terrace for a breath of fresh air when Harry suddenly emerged from the VIP room. 'You look freezing,' he said as he tried to light his cigarette in the wind. 'Do come and join my party.'"

At that time the newspapers were full of the princes' nightclub antics as they partied till dawn, drank the bars dry and danced with page-three girls.

"I was welcomed in and I was trusted," says Nicholl, adding that she has never abused this trust although she admits she treads a narrow line, trying to please editors looking for splashes and staying in the princes' good books. "I have only had one comeback. I have been told Prince William was surprised that I found out about his first love, Rose Farquhar.

"I was never part of the glosse posse, but I managed to infiltrate the inner circle by going to the clubs. I saw a lot of the princes and they were really great boys with the same problems we all have, the same doubts and fears about love, the future, passing exams.

"I have tried in my book to paint a sympathetic picture of them, a warm portrait. I think they are really nice boys."

The story of Prince William, the future king, lies at the heart of the book. He dislikes being the centre of attention, as illustrated in this poignant story. He was in Patagonia in his gap year, sitting around the fire with a group of friend discussing their futures.

William said: "I don't have much choice about my future. One day I will be king and, to be honest, I'm not much interested in that at all at the moment."

The people Nicholl spoke to, who were with him that night, said it broke their hearts.

But she is buoyant about his relationship with Kate Middleton and the book is full of intimate revelations of their lives together, particularly when they were at university in Edinburgh where they enjoyed the privacy of their own home.

"In their own house they were like any other couple in love. During the cold winter evenings they would spend their evenings watching DVDs and ordering in pizzas."

William needs a friend and a confidant and Nicholls believes Kate is perfect. "She knows when to back off when he gets claustrophobic. He trusts her and she has never let him down.

"She has proved herself time and again. She only spoke out once, and that was a story I broke, about her not having a job. The queen believes that whoever comes to the throne has to have an understanding of what life really is like. Ironically, Kate with her sheltered rich background has less idea than William.

"The relationship hasn't always been smooth but they are making the most of their freedom. At the moment they go on long weekends together and we (the press) know nothing about it. When they get married all that will stop."

According to Nicholl, William and Kate have a pact. A member of their inner circle explains: "William told Kate she was the one, but he was not ready to get married. He promised her his commitment and said he would not let her down, and she in turn agreed to wait for him."

Kate apparently set her cap at William before she left school. "This is the story I got from some of the girls from her school," Nicholls tells me. "The other girls were going out with local boys, drinking vodka, smoking behind the bicycle shed, but Kate had a picture of William on her wall and said she was waiting for him."

Chelsy is another story.

"I don't think Harry is going to end up with Chelsy," Nicholl says. "They are very on and off. Chelsy has decided she wants to come back to Africa, which was never the plan."

Nicholl implicitly trusts her sources, but admits to leaving out stories that would be harmful to the princes. However, for royal watchers there are a few chilling tales.

"One afternoon the Princess of Wales was lying on the sofa with Harry having tea with her friend astrologer Simone Simmons," Nicholl writes in the book. "Dressed casually in jeans and a pale blue cashmere sweater, her long legs tucked beneath her, the princess leaned in trying not to wake her son from his nap. 'What do you mean an accident? Who is in the car, Simone? You have to tell me,' she whispered urgently."

The boys have grown up. William has abandoned the line, "I'm a prince, wanna pull?" They take their careers in the armed forces seriously and have made peace with their stepmother.

"William and Laura, Camilla's daughter, used to have the most awful rows. When Charles phoned Camilla, Laura would scream down the phone, 'Why don't you leave our family alone?' But William was fascinated by Camilla. When they met, they got on very well. Harry viewed her suspiciously. He just didn't trust her. The turning point came when Harry was caught smoking weed and Camilla told Charles to back off because it was a passing phase."

The real tragedy of the royal princes is not that their privacy is punctured, but that their intellectual curiosity is blunted. Dr Declan Quigley, a lecturer in anthropology at St Andrews University, recalls Prince William once fell asleep during a talk on the fundamental principles of kingship. "I wish William had had the wit to ask me questions since I was passionate about the subject."

• William and Harry, Random House Struik, R215