People like my sister don’t just disappear

That night in the restaurant, Danny successfully argued to Ali that they needed a night out away from the children and the stress. "I know he was right but I felt guilty just the same," she says.

What happened to Claudia, 35, a chef at York University's Goodricke college, is a mystery. Lovely, privately educated, middle-class girls don't just vanish and certainly not from the genteel streets of York. Little wonder that the police - who would normally hold back for two or three days on the basis that most missing people reappear of their own accord - went to work on the case straight away.

"They could see that Claudia was not the sort of person who would just go off and not tell anyone," says Ali. "As soon as I heard she had not turned up at work I knew it was serious. She would have rung if she wasn't coming in, she hated to let anyone down."

The investigation into Claudia's disappearance has turned into the biggest operation in years for North Yorkshire police. More than 1,000 witness statements have been taken and 240 houses and 1,000 student rooms searched. A £10,000 reward for information has been put up by the Crimestoppers charity.

Claudia's serenely smiling face adorns hundreds of missing-person posters around the city streets - disconcerting for her parents who, although separated, still live in York - and it also appears on beer mats in pubs around Heworth, the area where she lived. The police are now suggesting a "complicated" love life might hold the key to her disappearance.

Her last known movements give no clue as to what was to come. Having worked an early shift, Claudia finished around 2pm and was caught on CCTV heading home. That evening - Wednesday, March 18 - she spoke to her mother and father from her two-bedroom cottage at around 8pm. At 9.12pm a friend sent her a text, though it is not known whether she read it.

Next morning she did not turn up for work at 6am, nor that evening to meet a friend, Suzy Cooper, in a local pub. Cooper assumed Claudia had fallen asleep as she'd been up so early, but when she still could not reach her on Friday morning she telephoned Claudia's father Peter, a solicitor, who had a spare key to her house.

He let himself in to find it empty. Claudia's passport and bank cards were there, but her rucksack containing her chef's whites was missing, suggesting she had left the house for work as usual at 5.15am. "It was before the clocks went forward, so it would have been dark," says Ali. "Or it's possible she went missing the evening before. We simply don't know."

No one seems to have heard or seen anything, save for a cyclist who was passing a bridge on Claudia's route to work and recalls seeing a man and a woman arguing. The woman was apparently wearing a blue jacket, similar to one owned by Claudia. The man, who was smoking, was wearing a dark hooded top and combat trousers.

The scene on Melrosegate bridge was reconstructed on Crimewatch 12 days ago - and shown alongside CCTV footage of a man near Claudia's house at 5.50am on the Thursday - but there has been no response from the people in question. "We can't understand it," says Ali. "When there's been so much publicity in York, you'd think people would come forward so they could be eliminated." However, she concedes that they may have not seen the programme, or may have simply forgotten they were there that day. "We just don't know what to think," she says.

Until now Ali has left it to her father to do the talking on the family's behalf. She has two boys, Luke, 4, and Joshua, 10 months, and has not revealed publicly that she is Claudia's sister because she could not face the thought of dropping Luke off at nursery only for people to say: "I'm sorry about your sister . . ." She dreaded Luke hearing and asking: "What about Auntie Claudia, Mummy, what?"

"But the alternative has been just as bad," she says. "Dropping him off and being asked how I am and smiling and saying, 'I'm fine'. I can't go on pretending everything is all right."

Telling Luke has been one of the hardest parts of the nightmarish situation. Ali consulted a child psychologist for advice on how to break the news: "I wanted to get it right. Danny and I explained the police had come to see us because we didn't know where Claudia was and they were helping us to look for her. He said, 'Are the police using binoculars, Daddy?'

"I don't want my anxiety to rub off on the boys. I am lucky I have them to distract me, but when I have put them to bed I can't stop my mind coming back to it. I lie in bed thinking about Claudia. My great fear is they will grow up not knowing her. Luke might have some vague memory of her, but Joshua will have nothing at all."

The sisters shared what sounds like an idyllic English childhood, growing up in a comfortable home in Malton, North Yorkshire. "Mum was delighted at having two daughters, she loves ballet and she had visions of us dancing, but we wanted to wear trousers and climb trees," says Ali. "When I was eight and Claudia was five we started riding. I was keen but she was fanatical. She'd spend all day at the stables riding, or mucking out or cuddling the horses.

"We used to look after the donkeys from Scarborough beach in the winter and Claudia had her own horse, Flash. She was always a bit more daring than me - she'd climb a bit higher or ride a bit faster. We were typical sisters. She'd pull my hair or lock me in my bedroom or whatever and I'd call for Mum, but once she'd sorted it out we'd play together all day long."

When Ali went to Sheffield University to study geography, Claudia would visit at weekends. But she was less academic and left school at 16. She wanted to take a course in travel and tourism at college but opted for catering because a friend was doing that course. "She didn't fancy doing A-levels," says Ali. "Mum and I were always saying, you could do better, go for a job as head chef, but she wasn't ambitious, she was happy with her life."

Claudia was a bridesmaid at Ali's wedding; when Joshua was born she was at the hospital within hours. "We didn't see each other that often but she was there when it mattered," says Ali. "We last spoke about 10 days before she disappeared. She was going to come down for the weekend but changed her mind as she wanted to see some friends. She said she had lots of time off over Easter and she'd come to see us - but by that time she was gone."

The possibility that Claudia is dead has to be faced: the police have already talked to the family about Jenny Nicholl, a 19-year-old who went missing from home in North Yorkshire in 2005. A 48-year-old man with whom she had been having an affair was found guilty of her murder, even though a body was never found. The police presumably see possible parallels with the Nicholl case and have been probing Claudia's private life. Their allusions to her "complicated" love life have given rise to speculation that she had a secret lover, perhaps a married man. Ali says her sister made "unfortunate" choices of boyfriends.

"I'm someone who's settled, with a young family, and that's what I wanted for her," she says, "but Claudia always seemed to pick people with issues or problems. I remember once she was supposed to be going on holiday to Florida with a boyfriend and when they got to the airport he wasn't allowed to get on the plane because it turned out he'd been charged with grievous bodily harm at some point and she didn't know anything about it.

"I thought, oh, no, not again. Over time she stopped talking to the family about that side of her life because she thought we'd shoot her down. Her friends thought she'd told them everything, but when something like this happens it makes you wonder. Somebody must know something. People don't just vanish into thin air."

Every so often Ali calls Claudia's mobile. "I know that doesn't seem to make much sense but I'm always hoping there might be an answer," she says.