A young mother was charged with her six-week-old baby's death after she had dosed it with barbiturates in an attempt to make it sleep. On the morning of the trial, the police forensic expert realised that a decimal point had appeared in the wrong place in his report, multiplying the amount of the drug supposedly found in the baby's body by a factor of 10. The true level could not have killed the child. It had been a victim of cot death.
Despite these and other instances of scientists getting it wrong, including the cases of the Birmingham Six and the Maguire Seven, many lawyers continue to regard forensic evidence by the prosecution as unassailable.
Andrew Hall, a barrister at Doughty Street chambers, says that the commercial considerations of those marketing DNA testing techniques are partly to blame. 'It's not in their interest to suggest that this sort of evidence can be questioned.' he says.
In its evidence to the Royal Commission on Criminal Justice, Forensic Access, a company that provides forensic science services to defence lawyers, warns: 'There is a popular misconception that (forensic science) provides an especially pure and objective form of evidence.'
Forensic scientists may give evidence of opinion as well as fact, the company says. 'Dangers arise if the court, and perhaps the scientist himself, is unclear which is which.'
Dr Angela Gallop, the founder of Forensic Access and a former Home Office forensic scientist, says that scientific evidence should be probed to expose weaknesses or areas of reasonable doubt. 'A number of lawyers admit that they regard forensic evidence as a closed book,' she says.
The need for defence lawyers to challenge police forensic evidence is likely to increase since the quality of prosecution evidence may be on the decline. In 1991 police forces were given budgets to buy expert help. As a result, some forces have stopped using the Home Office Forensic Science Service and have started shopping around for the cheapest deal.
Dr Gallop says there is no shortage of laboratories keen to develop forensic science as a sideline. She claims to have come across three cases in the past year in which police evidence provided by a substandard laboratory was seriously flawed.
'In one case, the handling procedures were so poor that the scientist could well have manufactured all the evidence he claimed to have found,' she says.
In this instance Dr Gallop was able to persuade the police that its forensic evidence was 'so deeply flawed, the best thing to do was to forget all about it'. Despite this episode, she believes other police units may continue to use this laboratory.
Difficulties may arise outside the laboratory also. Careless investigative procedures can result in police unwittingly manufacturing evidence. Dr Gallop cites the case of a man accused of breaking into a newsagency who was found to have glass fragments on his clothing that matched those from the shop. She showed that the fragments could have been transferred from the hands or clothing of the arresting police officers.
In another case, a rape suspect was put in a police car in which the victim had been sitting a few hours earlier. In this case, the defence scientist was able to show that fibres found on the suspect's clothing could have been transferred from the car seat.
Dr Gallop says that only those with many years' experience in criminal work are capable of such insights. At present, however, forensic science is completely unregulated. 'Any Tom, Dick or Harry with a scientific background or just a plain brass neck can set themselves up as an expert,' she says.
The Law Society accepts that it is difficult for lawyers to know which experts are legitimate. Roger Ede, secretary to the society's criminal law committee, says: 'The problem is that as far as most solicitors are concerned, so long as a scientist has a white coat and a laboratory, he must be able to do everything.'
The society operates an expert witness service, but Mr Ede says that its vetting procedures are limited: 'We ask about qualifications, but we don't have the resources to make the sort of inquiries that we believe ought to be made.'
Many of the directories and bodies that provide details of expert witnesses face the same problem. Like Forensic Access, the Law Society is urging the Royal Commission to introduce a system of recognised qualifications and minimum standards for forensic scientists. The House of Lords Select Committee on Science and Technology also is expected to make recommendations in this area.
Dr Gallop wants to see changes in the Legal Aid Board's attitude towards solicitors who want to challenge prosecution forensic evidence. 'At worst,' she says, 'the legal aid authorities may withhold funding altogether, because, in their lay view, the prosecution case is unassailable.'
She cites the case of a rape suspect who was refused funding for an independent forensic report. Fortunately, his family was able to raise the money for the tests. The prosecution failed to prove its case and the man was acquitted at trial.
In other cases, the Legal Aid Board may be prepared to pay only for a cheaper, local laboratory. 'Whatever the board assumes, proper forensic scientists are thin on the ground, and there is often no expert advice to be had locally,' Dr Gallop says.
She also claims that the board may be referring firms to the wrong type of expert, suggesting, for example, an analytical chemist for a strangling case. 'A case such as this needs a forensic biologist who is experienced in crime- scene investigation,' she says.
George Ritchie, a legal adviser to the Legal Aid Board, denies that specific recommendations are made about which expert to use. 'We do have to take cost into account and will sometimes suggest a cheaper expert,' he says. 'It is a difficult issue. But if you want to travel from A to B and a Mini will do, why pay for a Rolls-Royce?'
He says that the board's area committees, made up of practising lawyers, decide on funding for forensic science work. 'Members of the committees can't be expected to know everything about everything,' he says. 'So the onus is on the solicitor to convince them that authority for funding should be granted.'
If an application is rejected, a solicitor can reapply with more information, he says.
Tony Edwards, a partner with T V Edwards, agrees that prior authority for funding can usually be obtained. 'The problem is that it is the most dreadful rigmarole,' he says. 'Collecting all the information can take two or three weeks, and often you don't have two or three weeks.'