The 33-year-old hairdresser from Woolwich, south-east London, was the first piece in the jigsaw puzzle that led Kent police to the gang who carried out Britain's largest ever cash robbery and made off with £53m from the Securitas depot in Tonbridge, Kent. Yesterday, five of the gang at the centre of the conspiracy were jailed at the Old Bailey.

Hogg had helped to disguise four of the robbers, using latex, false hair, grease paint and modelling wax, so that they could impersonate police officers without being recognised later. The prosthetic masks were well-made and effective and none of the Securitas staff or the manager and his family who were kidnapped at gunpoint by the robbers were able to identify them. But within 24 hours of the lorry driving away with the looted money, there was a knock at Hogg's door.

A tip-off to the police had pointed them in the direction of Hogg's home and her place of employment, a hairdressers in Forest Hill, south-east London, called Hair Hectik. She had taken two weeks off work to build the masks for the men and had made an expedition to several theatrical goods shops in Covent Garden, central London, to buy the necessary equipment. She spent hundreds of pounds. The men needed the disguises because they were planning to impersonate police officers and thus could not wear the traditional robber's disguise of a ski mask or balaclava. It was the beginning of a frightening two years for the woman who had once worked on makeup counters at Harvey Nichols and Selfridges.

The detectives who first interviewed Hogg found a flustered young woman clearly out of her depth in the midst of such a huge crime, one to which 100 officers had been assigned, three times as many as to an ordinary murder case. She denied having any knowledge that any robbery was about to take place. "I have committed no crime so far as I know," she said in a statement handed to the police by her lawyer. "I may have done some work innocently without any knowledge of how my work would be used. I would like to assist police further ... but I am terrified as to what may happen to me and my family if I say too much."

As the daughter of a police officer, she was anxious to help. She told the police that she had assumed that the men needed the masks for a music video, perhaps, or some theatrical performance, and that she met them through a mutual friend. She said that she would certainly never have embarked on the work if she had known anything illegal was involved. The police were not convinced and she was charged with conspiracy to commit robbery and conspiracy to kidnap.

When the trial started in court eight of the Old Bailey last July, Hogg was clearly in a very distressed state. She shook as she went into the dock alongside her seven co-defendants, and was weeping, anxious and breathless. Some days she was too ill to attend court, and there were rumours of a breakdown. Eventually, she was given permission to sit outside the dock with her mother, as she claimed she was being intimidated by the other defendants. Four of them - Lea Rusha, Roger Coutts, Stuart Royle and Jetmir Bucpapa - knew that she would be able to identify them.

Then, halfway through the trial, came a dramatic twist. Her legal team approached the Crown Prosecution Service and said that she was prepared to give evidence for the prosecution. It was agreed that all charges should be dropped against her. This is known as "going QE" (Queen's evidence) or, more familiarly, "talk and walk".

The decision to call her as a witnesss was made "in the public interest". While the CPS's Roger Coe-Salazar said later that they were confident of securing convictions without Hogg, it was soon apparent that her evidence was dynamite.

She was adamant that she had disguised the men and denied that she was giving evidence to protect herself. There were angry clashes in the court. One of the defendants, Royle, who had sacked his lawyers and was defending himself, told her that "I am going to try and be as nice as I can". He admitted she had cut his hair at Hair Hectik shortly after he had had it dyed, but claimed that that was their only contact, and that she had never made a mask for him. "You trimmed my hair and said it looked nice," he told her as the cat and mouse game in the court developed. "You said I was a gentleman for helping you with your bags." She agreed that the new colour suited him. But when he claimed that she must have been at a different bungalow from his when she fitted the men with their disguises, she shot back: "Bless you, Stuart, but it was that bungalow. I did a face cast of you, Stuart, a nose was made for you. I am sorry but that is the bungalow that I went to. You offered me a drink and a cake or a biscuit ... You are absolutely lying and you are lying by saying the makeup wasn't done at your mum's bungalow."

The trial judge, Mr Justice Penry-Davey, had to intervene when they clashed again. "I'm sorry but the lies that come out of his mouth are ridiculous," she told the court, adding to the judge: "I am sorry, my lord, but he is talking absolute rubbish. He might as well be talking out of his bottom." The judge warned her to watch her language.

She gave a detailed description of how she had made the disguises. She told how one conspirator, who was black, had complained that she was making him look like a "black Father Christmas". She tried to change his appearance further, using her bra straps to pull his eyes sideways and inserting teats from a baby's bottle into his nostrils. She made bald caps, fake noses, fake chins. After Hogg had delivered her detailed and damning evidence, the prosecution's case was made. The jury may have taken more than a week to reach their verdicts but there can be little doubt that she played a very crucial part in helping them reach their decisions.

But there are at least three key members of the gang who are still at large (along with more than half of the £53m stolen), and wanted by the police. The alleged ringleader, a cage fighter, is believed to be in Morocco. If they could be brought back to the UK, Hogg would again be the prosecution's star witness. There are rumours that there is a contract out on her life - a £7m contract, according to some newspapers - but then there are always such rumours when someone has to assume a new identity for their own safety, which is Hogg's fate. She currently has 24-hour armed police protection, will be under the witness protection for the foreseeable future and is believed to be planning to radically alter her appearance.

The Witness Protection Scheme is designed to look after people who have essential evidence about some of the most serious offences, and who therefore face a real threat to their safety. Most of these protected witnesses are supergrasses - criminals who have turned on their associates in order to save their own skins. But some are simply innocent bystanders, or people on the fringes of gang culture.

Individuals living under the scheme have to trade in their old life for a new one - moving house (usually going to another part of the country, and sometimes as far away as Australia, Canada or New Zealand), and changing their identity, with medical records and educational qualifications under a new name. They are also given a lump sum of money to help them start their new life.

Hogg will be very well looked after by the officers assigned to her. Given her importance to any future trial, the police will be taking absolutely no risks that anyone can get to her - and already the press has been instructed not to publish her photograph without obscuring her eyes. But her life will not be easy.

Danielle Cable, whose evidence helped convict the man who murdered her fiance, Stephen Cameron, has been living under the scheme since 1998. In 2000, she told the Daily Mail that she was only given a few minutes to pack her bags and leave her old home - a trauma from which she was still recovering. She hadn't seen two of her brothers since she was relocated, she told the reporter, and was unable to see her mother for four months. Two years after she went into hiding, her phone calls were still strictly limited, she said - she could only make calls from untraceable locations - and it took six weeks for letters to reach her. "I have lost twice," she said, "Stephen and my old life."

In Britain, the police have a very good record of protecting their witnesses, although some have claimed that, once the evidence has been given, they are treated with less care and respect.

A number of people who gave evidence against former criminals have been killed but these were old-fashioned "grasses" who were not under the protection of the police. People such as Alan "Chalky" White, who ended up at the bottom of Cotswold Water Park in Gloucestershire in 1989 after giving evidence against fellow criminals. But witness protection does not guarantee safety. In 1999, the IRA supergrass Martin McGartland survived an attempt on his life after being shot six times in the stomach while living under an assumed identity in Whitley Bay.

More recently, in 2006, the issue came up in court in the wake of the case of Giles van Colle, who was killed by a man against whom he was due to give evidence. A court awarded £50,000 damages to his family after the judge ruled that the police had failed in their duty to protect him. Mrs Justice Cox ruled that, in failing to prevent Van Colle's death, the chief constable of Hertfordshire had violated articles two and eight of the European convention of human rights covering the right to life.

The numbers of witnesses who need protection grows every year - probably because of the increase in gang-related and organised crime - with hundreds of families moved every year. While it is relatively easy for an individual to disappear without trace, the way they can be found is through family members. Sometimes family members are moved, too - but police have to weigh up the risks: how many members of an extended family can be given a new identity, and how many of them will be able to keep their mouths shut?

Although giving evidence in these circumstances is terrifying, the truth is that most witnesses and informants are not pursued by the people against whom they have given evidence. Professional criminals might like the satisfaction of exacting vengeance but they also know that any action they took would place them at a far greater risk than they already were and would ensure that they probably spent most of the rest of their life in jail. But this can be little comfort to Hogg, as she starts her new life today.