CATHY came in from work to find the telephone ringing. She left the house and was at the police station in 20 minutes. Her client, a middle-aged man with the mental age of a child, was waiting to be questioned about a rape that had occurred the night before.

The man had told the police that he did not need a solicitor, but Cathy arranged one for him anyway. She did not leave the police station until 10.15pm, after being there for more than five hours, carefully explaining to him everything that was going on.

Cathy is a member of a pioneering scheme set up by the social services in Greenwich, south London. She helps people with learning difficulties who are being interviewed by the police - her formal title is 'appropriate adult', a label taken from the code of practice that was introduced with the Police and Criminal Evidence Act (Pace) in 1984.

The code is based on the premise that 'intellectually and socially impaired' people may not be able to defend their own interests, and requires the police to call an appropriate adult when they think that someone may be vulnerable, and therefore at risk. Judges can exclude confessions made in the absence of an appropriate adult, so it is in the interests of the police, as well as the suspect, that such a person is at the interview. Their task involves advising the accused, ensuring that the interview is conducted properly and enabling communication between the two parties.

According to Pace, the appropriate adult should ideally be a relative or guardian of the suspect. But they are not always available, and since the suspects may come from families in which literacy is low, their relatives often end up saying nothing in the interview even if they do attend. Solicitors cannot act in two capacities, so the professionals who are best suited for the role are social workers and psychologists.

But a psychologist is not generally on call in the middle of the night, and a social worker may have too great a workload to be able to offer the time. 'One woman I know spent seven hours in a station charged with a very minor offence, handling a stolen credit card,' says Isabel Clare, a psychologist. 'No social worker was going to drop something like child care to sit in on an interview with her.'

Ultimately the police can use anybody who is a 'responsible adult' and is not employed by them. On one occasion a man was brought in off the street to act as an appropriate adult; he was deemed to be responsible by virtue of the fact that he had a clean driving licence.

The Greenwich scheme provides a pool of appropriate adults, trained specifically for the job, who can be called on by the police at any time. It has been in operation for 18 months, ensuring back-up for social workers when they are unable to take the calls themselves. Up to eight volunteers are involved, all are either carers or workers in organisations such as Mencap and Mind.

Cathy began working with the mentally handicapped when she was nursing. She is now employed by the local authority on a housing scheme for mentally ill people. 'I used to work with policemen when they did their community training,' she says. 'I realised how some of these people were going through an awful time when they got into trouble with the police. I just wanted to do what I could to redress that.'

Ian Blackie, a Greenwich social worker, says there used to be only about one call every three months from the police; now there are several a week: 'The police have lost a number of cases over the absence of an appropriate adult. It has taken solicitors and barristers a long time to realise what Pace requires. Custody officers have been rapped over the knuckles and are being careful.'

Protecting the rights of vulnerable people is not easy to square with police procedure and efficiency. Detective Inspector John Pearse, of the Greenwich police, says he has seen cases thrown out of court on the grounds of unreliable testimony, although he was confident of the guilt of the accused. 'No matter how fair we are, the defence can always argue that there should have been an appropriate adult present. An unscrupulous lawyer can make an 11th-hour attempt to recruit a psychiatrist who would say that his client had a low IQ which rendered his admissions inadmissable. We need a clear definition of what constitutes 'at risk' within the legal process,' he says.

Suspects are often not recognised as having learning difficulties until well after the police interview, when a psychologist or medical examiner is called by the defence. If the case is going to the Crown Court, the person may be in custody for weeks or months in unsuitable surroundings. Matters are made worse by restrictions on the availability of legal aid. Expert testimony from psychiatrists on levels of IQ, suggestibility and vulnerability - important indicators of whether a confession is reliable - is not cheap, and legal-aid boards are reluctant to pay.

'These arrests are frequently for quite serious offences,' says Mr Blackie. 'You get someone who is very vulnerable, someone who knows what they have done but doesn't know that it is wrong. We fall down miserably on our responsibility to these people. Sometimes I have felt quite bad going away from a police station thinking this poor bugger's going to be banged up for the weekend.'

Raymond Watkins, for instance, falsely confessed to an armed robbery, and spent five months in prison pending trial. He had an IQ of 70, and was at the bottom of the prison pecking order. He became a 'boxing bag' for prison inmates, being beaten up and victimised frequently. He ended up having to do menial work for 'prison bullies'. Mr Watkins was acquitted at his trial because his testimony was unreliable.

The Royal Commission on Criminal Justice, headed by Lord Runciman, will publish its report next year. Recent miscarriages of justice have given it plenty to consider: Jacqueline Fletcher was convicted of murdering a baby, who was later found to have been a cot death victim; Stefan Kiszko confessed to raping and killing an 11-year-old girl, although he could not have secreted the sperm found on the girl's clothes; and Wayne Darvell had his conviction for the murder of a sex-shop manageress overturned. All three had learning difficulties.

Campaigners hope that the Runciman commission will follow up the Greenwich scheme and propose a national body of trained 'appropriate adults'.