Murderer Harold Shipman was cool and collected throughout his police interrogation until he was confronted with evidence that he had altered computer records at his surgery.

Fortunately for those who could have been the doctor's future victims, Greater Manchester is one of several police authorities in the country which use software from Oxfordshire computer company Vogon.

In the first week of Shipman's arrest, the police downloaded everything from the computer system at The Surgery, Hyde, in Greater Manchester. The system Shipman was using was Microdoc, designed specifically for GPs. After his arrest, Vogon's software got to work extracting the evidence from the hard drive. Vogon marketing director Sandie Stevenson said: "When the police sergeant took the evidence from the computer he was writing down the dates when the records were altered and his colleagues across the room jumped up and down with glee.

"He had proved that the computer had been updated after those people had died."

According to Brian Whittle and Jean Ritchie, authors of Prescription For Murder, a book relating the Shipman story, when the GP was charged with the first three murders, he said nothing. But when the police confronted him with the computer evidence, he collapsed on the floor, sobbing and gibbering.

Mrs Stevenson said: "A computer makes a good witness. With a human being, no one knows how they are going to behave in court and they could be lying. Computers always tell the truth." Greater Manchester Police had not simply bought the equipment. Like their counterparts in the Customs and Excise, Inland Revenue and Serious Fraud Squad, they had also been trained by Vogon in how to use it.

You might think that when you delete a file from your computer, it has gone but you'd be wrong. Vogon's forensic services manager Clive Carmichael-Jones said: "The only way to destroy computer evidence is to incinerate the hard drive." In Hitchhiker's Guide to the Galaxy Vogon were the people intent on destroying Earth, but the company's 40-strong workforce prides itself on helping out when data has fallen into a black hole. Paedophiles are particularly easy to catch, according to Mrs Stevenson, because their obsession leads them to collect and store images in a systematic and orderly way. And they are reluctant to destroy their collection, so that even if they get rid of the computer, the police often find a tape or disk hidden in a drawer.

Vogon helped to catch Gary Glitter and also collected evidence against Bay City Roller Derek Longmuir, who last month admitted downloading four indecent Internet pictures on his computer.

Vogon started in 1985 when Mrs Stevenson's husband Gordon, a computer science graduate with a long and varied career in computing, was made redundant. He set up his own consultancy, specalising in data conversion, translating information from a mainframe computer into a form usable on a PC. Mrs Stevenson joined him in 1989. She said: "We discovered that from time to time there was a flaw in the tape and he wrote a program that would allow us to get the data. What was a sideline at first is now the main part of our business."

In 1997 they merged with Authentic, a management buy-out from computer software firm Dr Solomon, whose virus software was bought by Network Associates. Dr Solomon now owns a 25 per cent stake in Vogon.

The company has doubled in size since moving to the Talisman Business Centre, Bicester, from Wokingham in 1997.

Mrs Stevenson said there was increasing demand for their data recovery services as well as the forensic and investigation work although many public bodies are unable to fund computer investigations. She said: "It makes me mad when I hear policitians saying how terrible child pornography is, yet we could develop software to allow pornography to be tracked on computers in a matter of minutes, if we could get Customs and Excise and the police to say that they could pay for it."

She is also outraged by the ease with which white-collar criminals get away with computer fraud which would be relatively simple to detect. She points out that many prisoners study computing, sometimes to degree level. Vogon has investigated one computer used at Broadmoor, where the authorities admitted there was no funding to check the others. More mundanely, Vogon acted for a company which suspected that an ex-employee had stolen its customer database to start a rival business. The business saved was estimated at $10m.

On one occasion, the company was called in after the perpetrator had broken a computer disk in half yet the data was retrieved.

Sometimes the police don't know what they are looking for, but simply suspect something fishy is going on. Vogon's software can bypass all the operating systems and search the original binary code. It can look for names of drugs, guns or terrorist equipment, or terms used in financial fraud. Mrs Stevenson said: "We have an amazing spread of customers large companies and banks as well as public-sector bodies."

The usefulness of computers is underlined by an interesting statistic one gigabyte of computer memory can hold the same amount of data as 50 four-drawer filing cabinets. A typical modern computer stores several gigabytes.

The evidence-collecting methods are obviously useful to businesses which lose computer files or data by accident a major disaster nowadays.

Customers have included a pet food company whose recipes were lost when a flood affected computers in a basement and an accountant who dropped a laptop from his yacht though often Vogon is called in when the company's own IT staff are stumped, or when other companies have failed to retrieve the data. Mrs Stevenson said: "We get such a buzz out of hearing that what we have done has worked, whether it is a child custody case involving a paedophile, embezzlement or simply rescuing lost data. People are becoming more aware of these things but there is a long way to go.

"We call our forensic bulletin The Smoking Gun because if you saw a smoking gun after a crime, you wouldn't tamper with it, you would leave it to the experts. It's the same with computers. You shouldn't even switch it on, because booting it up could destroy some evidence."