We are sitting together in a sumptuous room in the Mandarin Oriental hotel in London’s Knightsbridge. The hair has grown back to its luxurious trademark thickness, allowing him to shake his head in bewilderment at the memory. “I had a wonderful view of an endless horizon, blue sky. A privileged life. And the music just wasn’t coming.”

Frustrated and anxious, on impulse he picked up his electric razor and shaved his head. “I needed some kind of rebirth, and it was cathartic, a slap in the face,” he says in his gentle Scots accent.

Following a number-one single, platinum album and sell-out British tour, not only his record company but his fans were waiting to see what he would do next. “I was living a dream. Cutting off my hair was an act that was real, in a very unreal situation. And it felt really good.”

Despite that, it took a life-changing experience and another five months – back home in Glasgow, staying with his parents – before Darius wrote a song that he felt really proud of. A tragic love ballad, you might assume it was written for an ex-girlfriend. But Darius did not write the song for a lost love – he wrote it for someone he fears he is about to lose. His father Booth Danesh, a consultant gastroenterologist, was diagnosed with cancer in February this year. Always close to his parents, and proud of his background – his father is Persian, his mother Scottish – Darius was devastated.

“I came home one day, and Mum and Dad were talking in the kitchen,” he tells me, re-living the moment. “Something was wrong, but I didn’t know what it was. Dad said, “Darius, have a seat.” And then he explained that he’d been for a check-up that day, and that it was cancer. Terminal cancer.

He stops and sighs heavily, and then goes on. “I thought I was going to die,” he says, eyes wide with shock. “I literally lost my breath. I froze. I couldn’t cry, and I couldn’t speak. It hit me in the stomach, I think I might have lurched forward. My father is everything to me. I am very much my father’s son. We all just held each other.

“All that I’d had hanging over me for six moths came to a head. I’d had a feeling Dad was ill since the last night of my tour at Hammersmith Apollo, a wonderful night. Both my parents were there, and when I was singing a ballad, in the shimmering lights, one of the little sparks shone on my father’s face, and I sang it directly to him. But afterwards, when I came out front to meet them, the crowds swept them in the opposite direction. Dad collapsed that night, although I didn’t know until the next day. From that night on, something hung over me. But I thought, “He’s a doctor, he saves people’s lives. I thought he would live forever.”

“It started as lymphatic cancer, and it had spread by the time they discovered it. His frustration was huge that he hadn’t self-diagnosed. He’s on the board of the Royal College of Physicians, an eminent man. It was very painful for him.”

Within the week his father had been transferred to the Royal Marsden Hospital in London, and his mother Avril, a GP, moved into the hospital too: “She slept beside him.” It was understandably, a desperate time for the whole family – Darius, 24, has two brothers, Aria, 20, and Cyrus who is nine – but two weeks later, out of the blue Darius got a phone call from an old friend.

“He told me, ‘I don’t even know how to say this. Mum and Dad went on holiday to South Africa. On Christmas Day they went for a drive and they were killed instantly in a car crash.’

Poor Guy. And I thought, My God! How can I allow myself to feel negative about Dad’s illness? By comparison, I realised just how lucky we were to be able to spend this time with him, to say all the things we wanted to say.”

The realisation inspired him to write Live Twice, the title track on his new album. In contrast to the problems he had had in writing, this song poured out of him. It’s a wistful, soaring expression of yearning, and the words of regret in the poignant ballad will reverberate with anyone who has loved and lost.

“I wanted to write a song for him that would describe exactly how I felt,” he says. “I played it to him on guitar. It was so moving for both of us. It was like a dam had burst. I realised what my writer’s block was about. I was worried about people’s expectations of me. And I realised that all that matters, really, is your family. And life is so transient, so short. So the songs flowed thick and fast, and I’d play a new one to him in hospital every other day.”

Given his cheesy beginnings on Popstars – the goatee beard and ponytail saw him ridiculed in the streets – then Pop Idol, Darius cuts a very different figure today. His youth belies a wisdom and fierce sense of loyalty that lies behind the earnest façade. He is a bright, sweet-natured man who idolises his father.

“He’s an incredible man, very charismatic, very dignified.” His huge, dark eyes shine with pride. “The son of a Persian diplomat, he was brought up in a palace, but in the Iranian revolution the family lost everything – there are stories of secret police and death threats – and he came to this country to study medicine.

“He arrived with nothing, but he has achieved so much. He invented the first kidney/liver machine for premature babies. He met my mother in Glasgow, she was one of his students. He says he fell in love the day he met her, and Mum is the warmest, most sensitive, gentle person. They’ve been married for 25 years.

“The cancer is very extensive and has corrupted a lot of his body – lymphs and bone marrow. There have been moments where we’ve almost lost him, but there’s an amazing feeling of hope. Mum is a radiant source of love for all of us, and Dad is such a fighter, he’s an inspiration.”

Darius thinks we have much to learn from children. “It’s amazing how resilient they are,” he says. “When my little brother Cyrus was going to see Dad in hospital, I warned him that Dad had lost all his hair. Cyrus put his arms around me, and looked up at me. He said ‘Darius, you know, it’s going to be all right.’”

And his eyes fill with tears, but he swallows hard and continues. “When we left, he said, ’Darius, we have to do something to cheer him up. We’ve got to shave our heads. And film it. And show him.”’

“So we went home and got Aria. But Cyrus said to me, ‘It’s taken months for your hair to grow back. Don’t shave yours – what if you have a new video to shoot?’”

“Can you believe it? Nine years old… So I said ‘OK, I’ll film you two.’ As I watched Cyrus take the clippers to his head, it was very strange, because it echoed exactly what I’d done nine months earlier. Then I filmed him as he clipped Aria’s hair off. Cyrus swept up all the hair into a bag and demanded that we all wear beanie hats to surprise Dad. He had it all planned.”

“We walked into the room and Cyrus said, ‘Dad, we’ve got you a present,’ and handed him the bag. He said, ‘We passed a wig shop, and got you a wig.’”

“Dad opened it, and all the thick brown hair had clumped together. And then Cyrus said, ‘looks like we’re going to need to get two more.’ And they both pulled their hats off. You should have seen the expression on my father’s face!” Darius beams, eyes still glistening.

“Dad couldn’t speak. And then it was, ‘Oh my God!’ And he pulled them both towards him, three eggheads, all together. Such a lovely moment. Then Dad’s eyes filled with tears, and he said, ‘Cyrus I am your father, but you teach me about hope and about joy, and about not worrying, and smiling and laughing and making jokes. And how important that is.’”

We both have to blow our noses at this point.

Darius confesses, “Although I couldn’t cry that first day, I’ve cried a lot since then. At first I would sob in private, I wouldn’t want to let Dad see. Then I realised that tears do wash away the pain, and to cry together is a beautiful experience. To kiss a tear on someone’s cheek is a beautiful thing.”

There has been one casualty of this traumatic period of his life. Darius and his girlfriend split up four months ago. They had been together for 18 months.

“I fell for her the day I met her, and we had a wonderful relationship,” he says, fondly. “She supported me through amazing times and through the hardest part. We had a great time. Although we’ve split, we still love each other very much. It was a very painful break-up, there was nobody else involved, but it was the right thing to do. It truly was mutual, but, in fact, I didn’t start the conversation. It’s a big time of change for me, and I think it’s important that I’ve been on my own. She is the most amazing woman, and I still love her very dearly.”

He says he is enjoying being single, and the gossip columns have been quick to link him to actress Natasha Henstridge who features in the video for Kinda Love, his new single, a song about being head over heels in love. He laughs: “I met her in LA when I was recording. I thought she’d be great for the video, so we had dinner in LA once, and then over here. Now she’s back in the US.”

“We had fun shooting the video, but that’s it. Besides, I’ve come out of a long relationship, I want to honour the love I had. It’s very important not to start anything on the rebound. I don’t take relationships lightly.”

I tell him that he has a very wise head for such young shoulders. He grins and says, “I’m lucky that I’ve experienced such a lot in such a short space of time. We all make mistakes, but I made mine in front of millions of people.” He rolls his eyes. “Everyone has the dodgy school photo, but I went through the most embarrassing stage of my life in front of the nation’s eyes. I was 19, and at the time I was really hurt. Looking back, I’m grateful. Any experience that allows you to come out a better man is invaluable. My parents gave me such a solid base. When I was determined to audition for Pop Idol, they told me I’d be like a lamb to the slaughter. But when I persisted, and asked for their unconditional support, they gave me their blessing.”

He has dedicated the dozen songs on the new album to his father, and credits him with the very fact it exists. “I wrote Devil in You, a Sinatra-type swing song, for him, because that’s his era. And when I played it, he took my hand and said, ‘You haven’t told me the plans for your album release.’ I said, ‘I’ve decided not to release this year. I want to spend every day with you.’”

“He said, ‘Darius, I fight to live to see the achievements of my children. You must pursue your dream. Now, not tomorrow. Today I fight to see Cyrus’ sports day, to see Aria graduate. And your songs are inspiring for me, don’t hide them away. I want to hear them on the radio, to see you in concert. How can I do that if you don’t release your album?’”

“So it’s not just about my writer’s block. I have him to thank for the whole album, and I owe it to him. It’s the real deal, and I’m very proud of it. I hope he is too.”