When Dr Eric Shepherd steps into the witness box, police witnesses and prosecuting counsel must feel their hearts sinking.One of the country's leading forensic psychologists, Dr Shepherd has fostered a reputation for tearing apart prosecution cases.

His name is now known not only in police station canteens and Crown Courts, but also amongst the wider public, thanks mostly to his work on the Cardiff Three and Bridgewater Four cases.Dr Shepherd's technique of studying an entire case from arrest to court appearance is laborious but effective.

In simple terms, Dr Shepherd describes his objective as 'trying to give the court an explanation of why a particular suspect, victim or witness would behave in a particular way'.

The approach is becoming increasingly prominent in the analysis of child witnesses.But assessing an individual for their intellectual functions is, as Dr Shepherd points out, 'but one part of the jigsaw'.

Essentially, forensic psychologists stand back and take a wide-angle view of all three main stages of a criminal prosecution - starting with the offence itself and the police investigation, followed by the police interviews with the suspect, and lastly studying the final police account as prepared for trial.'We take everything - all the paperwork - that a solicitor has on the case,' Dr Shepherd explains.

'Then we produce a graphic representation of what happened.

Eventually, we are able to see things which were probably not observable by just looking at it in a straightforward linear text.'The process of analysis involves studying a suspect's vulnerability and suggestibility and putting that into the context of what happened during police station interviews.It is that interview stage of the criminal justice process which is currently preoccupying the minds of nearly all criminal law solicitors.

And while Dr Shepherd has become to the police 'the man they love to hate' (according to the Police Review), he may soon turn into 'the man they can't forget' to the legal profession.In conjunction with members of the Law Society's criminal law committee, Dr Shepherd has just produced a large training package on police station skills for solicitors' representatives.Its publication could not have come at a more opportune time, with research from the University of Warwick purporting to show a woeful inability on the part of legal advisers to counteract dubious police interviewing tactics.But what really gave the profession a kick in the pants was the Legal Aid Board's decision that all non-qualified legal representatives (except trainee solicitors) should pass a test and be registered by early next year if they were to continue to receive payment from public funds.Dr Shepherd's expertise was commissioned and the end result was Police Station Skills for Legal Advisers.

At more than 1000 pages and including a couple of audio cassettes, it is certainly impressive.Dr Shepherd has had a varied career to date.

It began in Army intelligence about 25 years ago, where he specialised in interrogation and resistance to questioning.

While still in the services, he did a psychology degree at the University of Reading and went on to do a Ph D at London University.

He is also a polyglot, speaking Arabic, Russian and German.After leaving the armed forces, Dr Shepherd went into academia, and is still a senior lecturer in psychotherapy and counselling at Guy's Hospital in London.

But most of his time now is spent operating the consultancy, Investigative Science, which he launched two-and-a-half years ago with his wife, Anna.Based in the Oxfordshire village of East Hendred, the Shepherds devote about 90% of their work to defence clients while keeping a hand in the other side by doing 10% for the prosecuting authorities.Salubrious East Hendred is a world away from the distinctly unpleasant climate of police station cells and interview rooms.

But it seems that, mentally at least, Dr Shepherd is never that far removed from the rough and tumble of police interviews.For Dr Shepherd, there is one overriding problem with criminal investigations: confessions.

'The status of the confession in the UK has been the single greatest block upon the development of the police service as an investigative culture.

There are up to eight possible reasons why a person will confess, including being guilty.

But we have had a shibboleth of the confession which the judiciary has gone along with.

All these miscarriage of justice cases were symptomatic of the problem.'At the core of the problem, says Dr Shepherd, is something akin to police laziness in the worst cases, or simply a lack of resources in others, resulting in a reluctance to do real detective work.

'The rationale is that if they [the police] can get a confession quickly then they can stop digging - they don't have to do much more work.'But fault lies not just with the police and the judiciary.

Lawyers too have been complicit.

'All too many legal advisers have been sitting there trying to see whether there is fair play going on rather than actually advising.

They have either stepped aside or, in some cases, taken the police side.'What should be happening in the police station interview room, says Dr Shepherd, is a mimicking of the courtroom, with the police case in the role of prosecutors and the legal adviser as defence advocate.Indeed, Dr Shepherd temporarily casts humility to one side, forecasting that the training pack produced for the Society will eventually change the very atmosphere inside police stations.

If the training is adhered to, then 'the legal adviser [will be put] in a straightforward, unequivocal defence stance.

That must improve the quality of interviews.

The police will be getting feedback from the defence legal adviser and they will therefore have to be much better prepared when they come into the interview room.'